Apparatus for withdrawing permeate using an immersed vertical skein of hollow fibre membranes

ABSTRACT

An apparatus is described for withdrawing filtered permeate from a substrate contained in a reservoir at ambient pressure. The apparatus includes a plurality of membrane assemblies. Each assembly has a plurality of hollow fiber filtering membranes, immersed in the reservoir, at least one permeating header with the membranes sealingly secured therein, and a permeate collector to collect the permeate sealingly connected to the at least one permeating header and in fluid communication with lumens of the membranes. The membranes of each assembly extend generally vertically upwards from a first header during permeation. One or more sources of suction are provided in fluid communication with the lumens of the membranes of each assembly through the permeate collectors and apply sufficient suction to withdraw permeate from the lumens of the membranes. An aeration system for discharging bubbles assists in keeping the membranes clean. In other aspects, a method of removing fouling materials from the surface of a plurality of porous membranes includes providing, from within a membrane module, gas bubbles in a uniform distribution relative to the membranes. The bubbles move past the surfaces of the membranes to dislodge fouling materials from them. The membranes are arranged in close proximity to one another and mounted to prevent excessive movement.

FIELD OF THE INVENTION

[0001] This application is a divisional of Ser. No. 10/178,838 filedJun. 25, 2002; which is a continuation of Ser. No. 09/849,573 filed May4, 2001; which is a continuation of Ser. No. 09/507,438 filed Feb. 19,2000 issued as U.S. Pat. No. 6,294,039; which is a division of Ser. No.09/258,999, filed Feb. 26, 1999, issued as U.S. Pat. No. 6,042,677;which is a division of Ser. No. 08/896,517, filed Jun. 16, 1997, issuedas U.S. Pat. No. 5,910,250; which is a continuation-in-part applicationof Ser. No. 08/690,045, filed Jul. 31, 1996, issued as U.S. Pat. No.5,783,083 which is a non-provisional of provisional application SerialNo. 60/012,921 filed Mar. 5, 1996 and a continuation-in-part of Ser. No.08/514,119, filed Aug. 11, 1995, issued as U.S. Pat. No. 5,639,373.PCT/CA96/00536 was filed on Aug. 8, 1996, published as WO97/006880, andclaimed priority from U.S. Ser. Nos. 08/514,119 and 08/690,045. Thedisclosure of all the patents and applications listed in this paragraphare hereby incorporated by this reference to them as if they were fullyset forth herein.

BACKGROUND OF THE INVENTION

[0002] This invention relates to a membrane device which is animprovement on a frameless array of hollow fiber membranes and a methodof maintaining clean fiber surfaces while filtering a substrate towithdraw a permeate, which is also the subject of U.S. Pat. No.5,248,424; and, to a method of forming a header for a skein of fibers.The term “vertical skein” in the title (hereafter “skein” for brevity),specifically refers to an integrated combination of structural elementsincluding (i) a multiplicity of vertical fibers of substantially equallength; (ii) a pair of headers in each of which are potted the opposedterminal portions of the fibers so as to leave their ends open; and,(iii) permeate collection means held peripherally in fluid tightengagement with each header so as to collect permeate from the ends ofthe fibers.

[0003] The term “fibers” is used for brevity, to refer to “hollow fibermembranes” of porous or semipermeable material in the form of acapillary tube or hollow fiber. The term “substrate” refers to amulticomponent liquid feed. A “multicomponent liquid feed” in this artrefers, for example, to fruit juices to be clarified or concentrated;wastewater or water containing particulate matter; proteinaceous liquiddairy products such as cheese whey, and the like. The term “particulatematter” is used to refer to micron-sized (from 1 to about 44 μm) andsub-micron sized (from about 0.1 μm to 1 μm) filterable matter whichincludes not only particulate inorganic matter, but also dead and livebiologically active microorganisms, colloidal dispersions, solutions oflarge organic molecules such as fulvic acid and humic acid, and oilemulsions.

[0004] The term header is used to specify a solid body in which one ofthe terminal end portions of each one of a multiplicity of fibers in theskein, is sealingly secured to preclude substrate from contaminating thepermeate in the lumens of the fibers. Typically, a header is acontinuous, generally rectangular parallelpiped of solid resin(thermoplastic or thermosetting) of arbitrary dimensions formed from anatural or synthetic resinous material. In the novel method describedhereinbelow, the end portions of individual fibers are potted inspaced-apart relationship in cured resin, most preferably by “potting”the end portions sequentially in at least two steps, using first andsecond potting materials. The second potting material (referred to as“fixing material”) is solidified or cured after it is deposited upon a“fugitive header” (so termed because it is removable) formed bysolidifying the first liquid. Upon removing the fugitive header, what isleft is the “finished” or “final” header formed by the second pottingmaterial. Of course, less preferably, any prior art method may be usedfor forming finished headers in which opposed terminal end portions offibers in a stack of arrays are secured in proximately spaced-apartrelationship with each other.

[0005] The '424 patent required potting the opposed ends of a framelessarray of fibers and dispensed with the shell of a module; it was animprovement on two preceding configurations disclosed in U.S. Pat. Nos.5,182,019, and 5,104,535, each of which used frameless arrays andavoided potting the fibers. The efficiency of gas-scrubbing a '424 arraywas believed to be due, at least in large part, to a substantial portionof the fibers of the fibers in the array lying in transverserelationship to a mass of rising bubbles, referred to herein as a“column of rising bubbles”, so as to intercept the bubbles. Specificexamples are illustrated in FIGS. 9, 9A, 10 and 11 of the '424 patent.

[0006] A '424 “array” referred to a bundle of arcuate fibers thegeometry of which array was defined by the position of a pair oftransversely spaced headers in which the fibers were potted. In the '424array, as in the array of this invention, each fiber is free to moveindependently of the others, but the degree of movement in the '424 isunspecified and arbitrary, while in the vertical skein of thisinvention, movement is critically restricted by the defined length ofthe fibers between opposed headers. Except for their opposed ends beingpotted, there is no physical restraint on the fibers of a skein. Toavoid confusion with the term “array” as used for the '424 bundle ofarcuate fibers, the term “skein fibers” is used herein to refer toplural arrays. An “array” in this invention refers to plural,essentially vertical fibers of substantially equal lengths, the one endsof each of which fibers are closely spaced-apart, either linearly in thetransverse (y-axis herein) direction to provide at least one row, andtypically plural rows of equidistantly spaced apart fibers. Lesspreferably, a multiplicity of fibers may be spaced in a random pattern.Typically, plural arrays are potted in a header and enter its face in agenerally x-y plane (see FIG. 5). The width of a rectangularparallelpiped header is measured along the x-axis, and is the relativelyshorter dimension of the rectangular upper surface of the header; and,the header's length, which is its relatively longer dimension, ismeasured along the y-axis.

[0007] This invention is particularly directed to relatively largesystems for the microfiltration of liquids, and capitalizes on thesimplicity and effectiveness of a configuration which dispenses withforming a module in which the fibers are confined. As in the '424patent, the novel configuration efficiently uses a cleansing gas,typically air, discharged near the base of a skein to produce bubbles ina specified size range, and in an amount large enough to scrub thefibers, and to cause the fibers to scrub themselves against one another.Unlike in the '424 system, the fibers in a skein are vertical and do notpresent an arcuate configuration above a horizontal plane through thehorizontal center-line of a header. As a result, the path of the risingbubbles is generally parallel to the fibers and is not crossed by thefibers of a vertical skein. Yet the bubbles scrub the fibers. Therestrictedly swayable fibers, because of their defined length, do notget entangled, and do not abrade each other excessively, as is likely inthe '424 array. The defined length of the fibers herein minimizes (i)shearing forces where the upper fibers are held in the upper header,(ii) excessive rotation of the upper portion of the fibers, as well as(iii) excessive abrasion between fibers. The fibers of this inventionare confined so as to sway in a “zone of confinement” (or “bubble zone”)through which bubbles rise along the outer surfaces of the fibers. Theside-to-side displacement of an intermediate portion of each fiberwithin the bubble zone is restricted by the fiber's length. The bubblezone, in turn, is determined by one or more columns of vertically risinggas bubbles, preferably of air, generated near the base of a skein.

[0008] Since there is no module in the conventional sense, the mainphysical considerations which affect the operation of a vertical skeinin a reservoir of substrate relate to intrinsic considerations, namely,(a) the fiber chosen, (b) the amount of air used, and (c) the substrateto be filtered. Such considerations include the permeability andrejection properties of the fiber, the process flow conditions ofsubstrate such as pressure, rate of flow across the fibers, temperature,etc., the physical and chemical properties of the substrate and itscomponents, the relative directions of flow of the substrate (if it isflowing) and permeate, the thoroughness of contact of the substrate withthe outer surfaces of the fibers, and still other parameters, each ofwhich has a direct effect on the efficiency of the skein. The goal is tofilter a slow moving or captive substrate in a large container underambient or elevated pressure, but preferably under essentially ambientpressure, and to maximize the efficiency of a skein which does so(filters) practically and economically.

[0009] In the skein of this invention, all fibers in the plural rows offibers, staggered or not, rise generally vertically while fixedly heldnear their opposed terminal portions in a pair of opposed, substantiallyidentical headers to form the skein of substantially parallel, verticalfibers. This skein typically includes a multiplicity of fibers, theopposed ends of which are potted in closely-spaced-apart profusion andbound by potting resin, assuring a fluid-tight circumferential sealaround each fiber in the header and presenting a peripheral boundaryaround the outermost peripheries of the outermost fibers. The positionof one fiber relative to another in a skein is not critical, so long asall fibers are substantially codirectional through one face of eachheader, open ends of the fibers emerge from the opposed other face ofeach header, and substantially no terminal end portions of fibers are infiber-to-fiber contact. We found that the skein of fibers, deployed tobe restrictedly swayable, were as ruggedly durable as they were reliablein operation.

[0010] The fibers are stated to be “restrictedly swayable”, because theextent to which they may sway is determined by the free length of thefibers relative to the fixedly spaced-apart headers, and the turbulenceof the substrate. When a large number of fibers is used in a skein, asis typically the case herein, the movement of a fiber adjacent to othersmay be modulated by the movement of the others, but the movement offibers within a skein is constricted. This system is therefore limitedto the use of a skein of fibers having a critically defined lengthrelative to the vertical distance between headers of the skein. Thedefined length limits the side-to-side movement of the fibers in thesubstrate in which they are deployed, except near the headers wherethere is negligible movement.

[0011] In the prior art, a vertical skein of fibers in a substrate istypically avoided due to expected problems relating to channelling ofthe feed. However, because the fibers are restrictedly swayable in a“bubble zone” as described herebelow, the fibers are substantiallyevenly contacted over their individual surfaces with substrate andprovide filtration performance based on a maximized surface which issubstantially the sum of the surface areas of all fibers in contact withthe substrate. Moreover, because of the ease with which the substratecoats the surfaces of the vertical fibers in a skein, and theaccessibility of those surfaces by air bubbles, the fibers may bedensely arranged in a header to provide a large membrane surface of upto 1000 m² and more.

[0012] One header of a skein is displaceable in any direction relativeto the other, either longitudinally (x-axis) or transversely (y-axis),only prior to the headers being vertically fixed in spaced apartparallel relationship within a reservoir, for example, by mounting oneheader above another, against a vertical wall of the reservoir whichfunctions as a spacer means. This is also true prior to spacing oneheader above another with other spacer means such as bars, rods, struts,I-beams, channels, and the like, to assemble plural skeins into a “bankof skeins” (“bank” for brevity), in which bank a row of lower headers isdirectly beneath a row of upper headers. After assembly into a bank, asegment intermediate the potted ends of each individual fiber isdisplaceable along either the x- or the y-axis, because the fibers areloosely held in the skein. There is essentially no tension on each fiberbecause the opposed faces of the headers are spaced apart at a distanceless than the length of an individual fiber.

[0013] By operating at ambient pressure, mounting the headers of theskein within a reservoir of substrate, and by allowing the fibersrestricted movement within the bubble zone in a substrate, we minimizedamage to the fibers. Because, a header secures at least 10, preferablyfrom 50 to 50,000 fibers, each generally at least 0.5 m long, in askein, it provides a high surface area for filtration of the substrate.

[0014] The fibers divide a reservoir into a “feed zone” and a withdrawalzone referred to as a “permeate zone”. The feed of substrate isintroduced externally (referred to as “outside-in” flow) of the fibers,and resolved into “permeate” and “concentrate” streams. The skein, or abank of skeins of this invention is most preferably used formicrofiltration with “outside-in” flow. Typically a bank is used in arelatively large reservoir having a volume in excess of 10 L (liters),preferably in excess of 1000 L, such as a flowing stream, more typicallya reservoir (pond or tank). Most typically, a bank or plural banks withcollection means for the permeate, are mounted in a tank underatmospheric pressure, and permeate is withdrawn from the tank.

[0015] Where a bank or plural banks of skeins are placed within a tankor bio-reactor, and no liquid other than the permeate is removed thetank is referred to as a “dead end tank”. Alternatively, a bank orplural banks may be placed within a bioreactor, permeate removed, andsludge disposed of; or, in a tank or clarifier used in conjunction witha bioreactor, permeate removed, and sludge disposed of.

[0016] Operation of the system relies upon positioning at least oneskein, preferably a bank, close to a source of sufficient air or gas tomaintain a desirable flux, and, to enable permeate to be collected fromat least one header. A desirable flux is obtained, and provides theappropriate transmembrane pressure differential of the fibers underoperating process conditions. “Transmembrane pressure differential”refers to the pressure difference across a membrane wall, resulting fromthe process conditions under which the membrane is operating.

[0017] The relationship of flux to permeability and transmembranepressure differential is set forth by the equation:

J=k▴P

[0018] wherein, J=flux; k=permeability constant;

[0019] ▴P=transmembrane pressure differential; and k 1/μRm where μviscosity of water and, Rm=membrane resistance.

[0020] The transmembrane pressure differential is preferably generatedwith a conventional non-vacuum pump if the transmembrane pressuredifferential is sufficiently low in the range from 0.7 kPa (0.1 psi) to101 kPa (1 bar), provided the pump generates the requisite suction. Theterm “non-vacuum pump” refers to a pump which generates a net suctionside pressure difference, or, net positive suction head (NPSH), adequateto provide the transmembrane pressure differential generated under theoperating conditions. By “vacuum pump” we refer to one capable ofgenerating a suction of at least 75 cm of Hg. A pump which generatesminimal suction may be used if an adequate “liquid head” is providedbetween the surface of the substrate and the point at which permeate iswithdrawn; or, by using a pump, not a vacuum pump. A non-vacuum pump maybe a centrifugal, rotary, crossflow, flow-through, or other type.Moreover, as explained in greater detail below, once the permeate flowis induced by a pump, the pump may not be necessary, the permeatecontinuing to flow under a “siphoning effect”. Clearly, operating withfibers subjected to a transmembrane pressure differential in the rangeup to 101 kpa (14.7 psi), a non-vacuum pump will provide adequateservice in a reservoir which is not pressurized; and, in the range from101 kPa to about 345 kPa (50 psi), by superatmospheric pressuregenerated by a high liquid head, or, by a pressurized reservoir.

[0021] The fibers are not required to be subjected to a narrowlycritical transmembrane pressure differential though fibers which operateunder a small transmembrane pressure differential are preferred. A fiberwhich operates under a small transmembrane pressure differential in therange from about 0.7 kPa (0.1 psi) to about 70 kpa (10 psi) may producepermeate under gravity alone, if appropriately positioned relative tothe location where the permeate is with-drawn. In the range from 3.5 kPa(0.5 psi) to about 206 kPa (30 psi) a relatively high liquid head may beprovided with a pressurized vessel. The longer the fiber, which greaterthe area and the more the permeate.

[0022] In the specific instance where a bank is used in combination witha source of cleansing gas such as air, both to scrub the fibers and tooxygenate a mixed liquor substrate, most, if not all of the airrequired, is introduced either continuously or intermittently, near thebase of the fibers near the lower header. The perforations through whichthe gas is discharged near the header are located close enough to thefibers so as to provide columns of relatively large bubbles, preferablylarger than about 1 mm in nominal diameter, which codirectionallycontact the fibers and flow vertically along their outer surfaces,scrubbing them. The outer periphery of the columns of bubbles define thezone of confinement in which the scrubbing force exerted by the bubbleson the fibers, keeps their surfaces sufficiently free of attachedmicroorganisms and deposits of inanimate particles to provide arelatively high and stable flow of permeate over many weeks, if notmonths of operation. The significance of this improvement will be betterappreciated when it is realized that the surfaces of fibers inconventional modules are cleaned nearly every day, and sometimes moreoften.

[0023] Because this system, like the '424 system, does away with using ashell, there is no void space within a shell to be packed with fibers;and, because of gas being introduced proximately to, and near the baseof skein fibers, there is no need to maintain a high substrate velocityacross the surface of the fibers to keep the surfaces of the fibersclean. As a result, there is virtually no limit to the number ofrestrictedly swayable fibers which may be used in a skein, the practicallimit being set by (i) the ability to pot the ends of the fibersreliably; (ii) the ability to provide sufficient air to the surfaces ofessentially all the fibers, and (iii) the number of banks which may bedeployed in a tank, pond or lake, the number to be determined by thesize of the body of water, the rate at which permeate is to bewithdrawn, and, the cost of doing so.

[0024] Typically, a relatively large number of long fibers, at least100, is used in a skein of restrictedly swayable fibers, the fibersoperate under a relatively low transmembrane pressure differential, andpermeate is withdrawn with a non-vacuum pump. If the liquid head,measured as the vertical distance between the level of substrate and thelevel from which permeate is to be withdrawn, is greater than thetransmembrane pressure differential under which the fiber operates, thepermeate will be separated from the remaining substrate, due to gravity.

[0025] Irrespective of whether a non-vacuum pump, vacuum pump, or othertype of pump is used, or permeate is withdrawn with a siphoning effect,it is essential that the fibers in a skein be positioned in a generallyvertical attitude, rising above the lower header. An understanding ofhow a vertical skein operates will make it apparent that, since fibersin a skein are anchored at the base of the skein by the lower header,the specific gravity of the fibers relative to that of the substrate ismaterial and will not affect their vertical disposition.

[0026] The unique method of forming a header disclosed herein allows oneto position a large number of fibers, in closely-spaced apartrelationship, randomly relative to one another, or, in a chosengeometric pattern, within each header of synthetic resinous material. Itis preferred to position the fibers in arrays before they are potted toensure that the fibers are spaced apart from each other precisely, and,to avoid wasting space on the face of a header; it is essential, forgreatest reliability, that the fibers not be contiguous. By sequentiallypotting the terminal portions of fibers in stages as described herein,the fibers may be cut to length in an array, either after, or prior tobeing potted. The use of a razor-sharp knife, or scissors, or othercutting means to do so, does not decrease the open cross-sectional areaof the fibers' bores (“lumens”). The solid resin forms a circumferentialseal around the exterior terminal portions of each of the fibers, openends of which protrude through the permeate-discharging face of eachheader, referred to as the “aft” face.

[0027] Further, one does not have to cope with the geometry of a frame,the specific function of which is to hold fibers in a particulararrangement within the frame. In a skein, the sole function of theheader spacing means is to maintain a fixed vertical distance betweenheaders which are not otherwise spaced apart. In a skein of thisinvention, there is no frame.

[0028] The skein of this invention is most preferably used to treatwastewater in combination with a source of an oxygen-containing gaswhich is bubbled within the substrate, near the base of a lower header,either within a skein or between adjacent skeins in a bank, for thespecific purpose of scrubbing the fibers and oxygenating the mixedliquor in activated sludge, such as is generated in the bioremediationof wastewater. It was found that, as long as enough air is introducednear the base of each lower header to keep the fibers awash in bubbles,and the fibers are restrictedly swayable in the activated sludge, abuild-up of growth of microbes on the surfaces of the fibers isinhibited while permeate is directly withdrawn from activated sludge,and excellent flow of permeate is maintained over a long period. Becauseessentially all surface portions of the fibers are contacted bysuccessive bubbles as they rise, whether the air is suppliedcontinuously or intermittently, the fibers are said to be “awash inbubbles.”

[0029] The use of an array of fibers in the direct treatment ofactivated sludge in a bioreactor, is described in an article titled“Direct Solid-Liquid Separation Using Hollow Fiber Membrane in anActivated Sludge Aeration Tank” by Kazuo Yamamoto et al in Wat. Sci.Tech. Vol. 21, Brighton pp 43-54, 1989, and discussed in the '424patent, the disclosure of which is incorporated by reference thereto asif fully set forth herein. The relatively poor performance obtained byYamamoto et al was mainly due to the fact that they did not realize thecritical importance of maintaining flux by aerating a skein of fibersfrom within and beneath the skein. They did not realize the necessity ofthoroughly scrubbing substantially the entire surfaces of the fibers byflowing bubbles through the skein to keep the fibers awash in bubbles.This requirement becomes more pronounced as the number of fibers in theskein increases.

[0030] As will presently be evident, since most substrates arecontaminated with micron and submicron size particulate material, bothorganic and inorganic, the surfaces of the fibers in any practicalmembrane device must be maintained in a clean condition to obtain adesirable specific flux. To do this, the most preferred use of the skeinas a membrane device is in a bank, in combination with agas-distribution means, which is typically used to distribute air, oroxygen-enriched air between the fibers, from within the skein, orbetween adjacent skeins, at the bases thereof.

[0031] Tests using the device of Yamamoto et al indicate that when theair is provided outside the skein the flux decreases much faster over aperiod of as little as 50 hr, confirming the results obtained by them.This is evident in FIG. 1 described in greater detail below, in whichthe graphs show results obtained by Yamarnoto et al, and the '424 array,as well as those with the vertical skein, all three assemblies usingessentially identical fibers, under essentially identical conditions.

[0032] The investigation of Yamamoto et al with downwardly suspendedfibers was continued and recent developments were reported in an articletitled “Organic Stabilization and Nitrogen Removal in MembraneSeparation Bioreactor for Domestic Wastewater Treatment” by C.Chiemchaisri et al delivered in a talk to the Conference on MembraneTechnology in Wastewater Management, in Cape Town, South Africa, Mar.2-5, 1992, also discussed in the '424 patent. The fibers were suspendeddownwardly and highly turbulent flow of water in alternate directions,was essential.

[0033] It is evident that the disclosure in either the Yamamoto et al orthe Chiemchaisri et al reference indicated that the flow of air acrossthe surfaces of the suspended fibers did little or nothing to inhibitthe attachment of microorganisms from the substrate.

SUMMARY OF THE INVENTION

[0034] It has been discovered that bubbles of a fiber-cleansing gas(“scrubbing gas”) flowing parallel to fibers in a vertical skein aremore effective than bubbles which are intercepted by arcuate fiberscrossing the path of the rising bubbles. Bubbles of an oxygen-containinggas to promote growth of microbes unexpectedly fails to build-up growthof microbes on the surfaces of the fibers because the surfaces are“vertically air-scrubbed”. Deposits of animate and/or inanimateparticles upon the surfaces of fibers are minimized when therestrictedly swayable fibers are kept awash in codirectionally risingbubbles which rise with sufficient velocity to exert a physicalscrubbing force (momentum provides the energy) to keep the fiberssubstantially free of deleterious deposits. Thus, an unexpectedly highflux is maintained over a long period during which permeate is producedby outside-in flow through the fibers.

[0035] It has also been discovered that permeate may be efficientlywithdrawn from a substrate for a surprisingly long period, in a singlestage, essentially continuous filtration process, by mounting a pair ofheaders in vertically spaced apart relationship, one above another,within the substrate which directly contacts a multiplicity of longvertical fibers in a “gas-scrubbed assembly” comprising a skein and agas-distribution means. The skein has a surface area which is atleast >1 m², and opposed spaced-apart ends of the fibers are secured inspaced-apart headers, so that the fibers, when deployed in thesubstrate, acquire a generally vertical profile therewithin and swaywithin the bubble zone defined by at least one column of bubbles. Thelength of fibers between opposed surfaces of headers from which theyextend, is in a critical range from at least 0.1% (percent) longer thanthe distance separating those opposed faces, but less than 5% longer.Usually the length of fibers is less than 2% longer, and most typically,less than 1% longer, so that sway of the fibers is confined within avertical zone of movement, the periphery of which zone is defined byside-to-side movement of outer fibers in the skein; and, the majority ofthe fibers near the periphery move in a slightly larger zone than onedefined by the projected area of one header upon the other. Though thedistance between headers is fixed during operation, the distance ispreferably adjustable to provide an optimum length of fibers, within theaforesaid ranges, between the headers. It has been found that for noknown reason, fibers which are more than 5% but less than 10% longerthan the fixed distance between the opposed faces of the headers of askein, tend to shear off at the face; and those 10% longer tend to clumpup in the bubble zone.

[0036] The terminal end portions of the fibers are securednon-contiguously in each header, that is, the surface of each fiber issealingly separated from that of another adjacent fiber with curedpotting resin. Preferably, for maximum utilization of space on a header,the fibers are deliberately set in a geometrically regular pattern.Typically permeate is withdrawn from the open ends of fibers whichprotrude from the permeate-discharging aft (upper) face of a header. Theoverall geometry of potted fibers is determined by a ‘fiber-settingform’ used to set individual fibers in an array. The skein operates in asubstrate held in a reservoir at a pressure in the range from 1 atm toan elevated pressure up to about 10 atm in a pressurized vessel, withoutbeing confined within the shell of a module.

[0037] It is therefore a general object of this invention to provide anovel, economical and surprisingly trouble-free membrane device, forproviding alternative to both, a conventional module having pluralindividual arrays therewithin, and also to a frameless array of arcuatefibers; the novel device includes, (i) a vertical skein of amultiplicity of restrictedly swayable fibers, together having a surfacearea in the range from 1 m² to 1000 m², preferably from 10 m² to 100 m²,secured only in spaced-apart headers; and (ii) a gas-scrubbing meanswhich produces at least one column of bubbles engulfing the skein. Askein includes permeate pans disposed, preferably non-removably, withina substrate held in a reservoir of arbitrary proportions, the reservoirtypically having a volume in excess of 100 L (liters), generally inexcess of 1000 L. A fluid component is to be selectively removed fromthe substrate.

[0038] It is a specific object of this invention to provide a membranedevice having hollow fibers for removing permeate from a substrate,comprising, a skein of a multiplicity of fibers restrictedly swayable inthe substrate, the opposed terminal end portions of which fibers inspaced-apart relationship, are potted in a pair of headers, one upperand one lower, each adapted to be mounted in vertically spaced apartgenerally parallel relationship, one above the other, within thesubstrate; essentially all the ends of fibers in both headers are openso as to pass permeate through the headers; the fibers in a skein have alength in the range from at least 0.1% greater, but less than 5% greaterthan the direct distance between opposed faces of the upper and lowerheaders, so as to present the fibers, when they are deployed, in anessentially vertical configuration; permeate is collected in acollection means, such as a permeate pan; and, permeate is withdrawnthrough a ducting means including one or more conduits and appropriatevalves.

[0039] It has also been discovered that skein fibers are maintainedsufficiently free from particulate deposits with surprisingly littlecleansing gas, so that the specific flux at equilibrium is maintainedover a long period, typically from 50 hr to 1500 hr, because the skeinis immersed so as to present a generally vertical profile, and, theskein is maintained awash in bubbles either continuously orintermittently generated by a gas-distribution means (“air-manifold”).The air-manifold is disposed adjacent the skein's lower header togenerate a column of rising bubbles within which column the fibers areawash in bubbles. A bank of skeins is “gas-scrubbed” with pluralair-tubes disposed between the lower headers of adjacent skeins, mostpreferably, also adjacent the outermost array of the first and lastskeins, so that for “n” headers there are “n+1” air-manifolds. Eachheader is preferably in the shape of a rectangular parallelpiped, theupper and lower headers having the same transverse (y-axis) dimension,so that plural headers are longitudinally stackable (along the x-axis).Common longitudinally positioned-linear air-tubes, or, individual,longitudinally spaced apart vertically rising air-tubes, service thebank, and one or more permeate tubes withdraw permeate.

[0040] It is therefore a general object of this invention to provide agas-scrubbed assembly of fibers for liquid filtration, the assemblycomprising, (a) bank of gas-scrubbed skeins of fibers which separate adesired permeate from a large body of multicomponent substrate havingfinely divided particulate matter in the range from 0.1 μm −44 μmdispersed therein, (b) each skein comprising at least 20 fibers havingupper and lower terminal portions potted spaced-apart, in upper andlower headers, respectively, the fibers being restrictedly swayable in abubble zone, and (c) a shaped gas-distribution means adapted to providea profusion of vertically ascending bubbles near the lower header, thelength of the fibers being from at least 0.1% but less than 5% greaterthan the distance between the opposed faces of the headers. Thegas-distribution means has through-passages therein through which gas isflowed at a flow rate which is proportional to the number of fibers. Theflow rate is generally in the range from 0.47-14 cm³/sec per fiber(0.001-0.03 scfm/fiber) (standard ft³ per minute per fiber), typicallyin the range from 1.4-4.2 cm³/sec/fiber (0.003-0.009 scfm/fiber). Thesurface area of the fibers is not used to define the amount of air usedbecause the air travels substantially vertically along the length ofeach fiber. The gas generates bubbles having an average diameter in therange from about 0.1 mm to about 25 mm, or even larger.

[0041] It is a specific object of this invention to provide theaforesaid novel gas-scrubbed assembly comprising, a bank of verticalskeins and a shaped gas-distribution means for use with the bank, in asubstrate in which microorganisms grow, the assembly being used incombination with vertically adjustable spacer means for mounting theheaders in vertically spaced apart relationship, and in open fluidcommunication with collection means for collecting the permeate; meansfor withdrawing the permeate; and, sufficient air is flowed through theshaped gas-distribution means to generate enough bubbles flowingupwardly through the skein, between and parallel to the fibers so as tokeep the surfaces of the fibers substantially free from deposits of livemicroorganisms as well as small inanimate particles which may be presentin the substrate.

[0042] It has still further been discovered that a system utilizing abank of vertical skeins of fibers potted in headers verticallyspaced-apart by spacer means, and deployed in a substrate containingparticulate material, in combination with a proximately disposedgas-distribution means to minimize fouling of the membranes, may beoperated to withdraw permeate under gravity alone, so that the cost ofany pump to withdraw permeate is avoided, provided the net positivesuction head corresponding to the vertical height between the level ofsubstrate, and the location of withdrawal of permeate, provides thetransmembrane pressure differential under which the fibers function inthe skein.

[0043] It is therefore a general object of this invention to provide theforegoing system in which opposed terminal end portions of skein fibersare essentially free from fiber-to-fiber contact after being potted inupper and lower headers kept vertically spaced-apart with spacer means,the skein being unconfined in a shell of a module and deployed in thesubstrate without the fibers being supported during operation except bythe spacer means which support only the headers; the headers beingmounted so that the fibers present a generally vertical profile yet arerestrictedly swayable in a zone of confinement defined by risingbubbles; means for mounting each header in open fluid communication withcollection means for collecting permeate, and, means for withdrawing thepermeate; and, shaped gas-distribution means adapted to generate bubblesfrom micron-size to 25 mm in nominal diameter, most preferably in thesize range from 1 mm to 20 mm, the bubbles flowing upwardly through andparallel to the fibers at a flow rate chosen from the range specifiedhereabove; whereby the fibers are scrubbed with bubbles and resist theattachment of growing microorganisms and any other particulate matter tothe surfaces of the fibers, so as to maintain a desirable specific fluxduring operation.

[0044] Still further, a low cost process has been discovered fortreating a multicomponent substrate under pressure ranging from 1-10 atmin a pressurizable vessel, particularly for example, an aqueous streamcontaining finely divided inorganic matter such as silica, silicic acid,or, activated sludge, when the substrate is confined in a large tank orpond, by using a bank of vertical skeins each comprising restrictedlyswayable unsupported fibers potted in headers in open fluidcommunication with a means for withdrawing permeate, in combination witha source of air which generates bubbles near the lower header.

[0045] It is therefore a general object of this invention to provide aprocess for maintaining relatively clean fiber surfaces in an array of amembrane device while separating a permeate from a substrate, theprocess comprising, submerging a skein of restrictedly swayablesubstantially vertical fibers within the substrate so that upper andlower headers of the skein are mounted one above the other with amultiplicity of fibers secured between said headers, the fibers havingtheir opposed terminal portions in open fluid communication withpermeate collecting means in fluid-tight connection with said headers;the fibers operating under a transmembrane pressure differential in therange from about 0.7 kPa (0.1 psi) to about 345 kPa (50 psi), and alength from at least 0.1% to about 2% greater than the direct distancebetween the opposed faces of upper and lower headers, so as to present,when the fibers are deployed, a generally vertical skein of fibers;

[0046] maintaining an essentially constant flux substantially the sameas the equilibrium flux initially obtained, indicating ′ that thesurfaces of the fibers are substantially free from further build-up ofdeposits once the equilibrium flux is attained; collecting the permeate;and,

[0047] withdrawing the permeate.

[0048] It has still further been discovered that the foregoing processmay be used in the operation of an anaerobic or aerobic biologicalreactor which has been retrofitted with the membrane device of thisinvention. The anaerobic reactor is a closed vessel and the scrubbinggas is a molecular oxygen-free gas, such as nitrogen.

[0049] It is therefore a general object of this invention to provide anaerobic biological reactor retrofitted with at least one gas-scrubbedbank of vertical skeins, each skein made with from 500 to 5000 fibers inthe range from 1 m to 3 m long, in combination with a permeatecollection means, and to provide a process for the reactor's operationwithout being encumbered by the numerous restrictions and limitationsimposed by a secondary clarification system.

[0050] A novel composite header is provided for a bundle of hollow fibermembranes or “fibers”, the composite header comprising a molded,laminated body of arbitrary shape, having an upper lamina formed from a“fixing” (potting) material which is laminated to a lower lamina formedfrom a “fugitive” potting material. The terminal portions of the fibersare potted in the fugitive potting material when it is liquid,preferably forming a generally rectangular parallel-piped in which theopen ends of the fibers (until potted) are embedded and plugged, keepingthe fibers in closely spaced-apart substantially parallel relationship.The plugged ends of the fibers fail to protrude through the lower (aft)face of the lower lamina, while the remaining lengths of the fibersextend through the upper face of the lower lamina. The upper laminaextends for a height along the length of the fibers sufficient tomaintain the fibers in the same spaced-apart relationship relative toone and another as their spaced-apart relationship in the lower portion.If desired, the composite header may include additional laminae, forexample, a “cushioning” lamina overlying the fixing lamina, to cushioneach fiber around its embedded outer circumference; and, a “gasketing”lamina to provide a suitable gasketing material against which thepermeate collection means may be mounted.

BRIEF DESCRIPTION OF THE DRAWINGS

[0051] The foregoing and additional objects and advantages of theinvention will best be understood by reference to the following detaileddescription, accompanied by schematic illustrations of preferredembodiments of the invention, in which illustrations like referencenumerals refer to like elements, and in which:

[0052]FIG. 1 is a graph in which the variation of flux is plotted as afunction of time, showing three curves for three runs made with threedifferent arrays, in each case, using the same amount of air, theidentical membranes and the same membrane surface area. The resultsobtained by Yamamoto et al are plotted as curve 2 (under conditionsmodified to give them the benefit of doubt as to the experimentalprocedure employed, as explained below); the flux obtained using thegas-scrubbed assembly of the '424 patent is shown as curve 1; and theflux obtained using the gas-scrubbed assembly of this invention is shownas curve 3.

[0053]FIG. 2 is a perspective exploded view schematically illustrating amembrane device comprising a skein of fibers, unsupported duringoperation of the device, with the ends of the fibers potted in a lowerheader, along with a permeate collection pan, and a permeate withdrawalconduit. By “unsupported” is meant ‘not supported except for spacermeans to space the headers’.

[0054]FIG. 2A is an enlarged detail side elevational view of a side wallof a collection pan showing the profile of a header-retaining step atopthe periphery of the pan.

[0055]FIG. 2B is a bottom plan view of the header showing a randompattern of open ends protruding from the aft face of a header whenfibers are potted after they are stacked in rows and glued togetherbefore being potted.

[0056]FIG. 3 is a perspective view of a single array, schematicallyillustrated, of a row of substantially coplanarly disposed parallelfibers secured near their opposed terminal ends between spaced apartcards. Typically, multiple arrays are assembled before beingsequentially potted.

[0057]FIG. 4 illustrates a side elevational view of a stack of arraysnear one end where it is together, showing that the individual fibers(only the last fiber of each linear array is visible, the remainingfibers in the array being directly behind the last fiber) of each arrayare separated by the thickness of a strip with adhesive on it, as thestack is held vertically in potting liquid.

[0058]FIG. 5 is a perspective view schematically illustrating a skeinwith its integral finished header, its permeate collection pan, and twinair-tubes feeding an integral air distribution manifold potted in theheader along an outer edge of the skein fibers.

[0059]FIG. 6 is a side elevational view of an integral finished headershowing details of a permeate pan submerged in substrate, the walls ofthe header resting on the bottom of a reservoir, and multiple air-tubesfeeding integral air distribution manifolds potted in the header alongeach outer edge of the skein fibers.

[0060]FIG. 7A is a perspective view schematically illustrating anair-manifold from which vertical air-tubes rise.

[0061]FIG. 7B is a perspective view schematically illustrating a tubularair-manifold having a transverse perforated portion, positioned byopposed terminal portions.

[0062]FIG. 8 is a perspective view of an integral finished header havingplural skeins potted in a common header molded in an integral permeatecollection means with air-tubes rising vertically through the headerbetween adjacent skeins, and along the outer peripheries of the outerskeins.

[0063]FIG. 9 is a detail, not to scale, illustratively showing a gasdistribution means discharging gas between arrays in a header, andoptionally along the sides of the lower header.

[0064]FIG. 10 is a perspective view schematically illustrating a pair ofskeins in a bank in which the upper headers are mounted by their ends onthe vertical wall of a tank. The skeins in combination with agas-distribution means form a “gas-scrubbing assembly” deployed within asubstrate, with the fibers suspended essentially vertically in thesubstrate. Positioning the gas-distribution means between the lowerheaders (and optionally, on the outside of skein fibers) generate masses(or “columns”) of bubbles which rise vertically, codirectionally withthe fibers, yet the bubbles scrub the outer surfaces of the fibers.

[0065]FIG. 11 is a perspective view of another embodiment of thescrubbing-assembly showing plural skeins (only a pair is shown)connected in a bank with gas-distribution means disposed betweensuccessive skeins′, and, optionally, with additional gas-distributionmeans fore and aft the first and last skeins, respectively.

[0066]FIG. 12 is an elevational view schematically illustrating a bankof skeins mounted against the wall of a bioreactor, showing theconvenience of having all piping connections outside the liquid.

[0067]FIG. 13 is a plan view of the bioreactor shown in FIG. 12 showinghow multiple banks of skeins may be positioned around the circumferenceof the bioreactor to form a large permeate extraction zone while aclarification zone is formed in the central portion with the help ofbaffles.

[0068]FIG. 14 illustratively shows another embodiment of the skein inwhich the permeate tube is concentrically disposed within the air supplytube and both are potted, near their lower ends in the lower header.Ports in the lower end of the air supply tube provide air near the baseof the skein fibres.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0069] The skein of this invention may be used in a liquid-liquidseparation process of choice, and more generally, in various separationprocesses. The skein is specifically adapted for use in microfiltrationprocesses used to remove large organic molecules, emulsified organicliquids and colloidal or suspended solids, usually from water. Typicalapplications are (i) in a membrane bioreactor, to produce permeate aspurified water and recycle biomass; for (ii) tertiary filtration ofwastewater to remove suspended solids and pathogenic bacteria; (iii)clarification of aqueous streams including filtration of surface waterto produce drinking water (removal of colloids, long chain carboxylicacids and pathogens); (iv) separation of a permeable liquid component inbiotechnology broths; (v) de-watering of metal hydroxide sludges; and,(vi) filtration of oily wastewater, inter alia.

[0070] The problem with using a conventional membrane module toselectively separate one fluid from another, particularly using themodule in combination with a bioreactor, and the attendant costs ofoperating such a system, have been avoided. In those instances where anunder-developed country or distressed community lacks the resources toprovide membrane modules, the most preferred embodiment of thisinvention is adapted for use without any pumps. In those instances wherea pump is conveniently used, a vacuum pump is unnecessary, adequatedriving force being provided by a simple centrifugal pump incapable ofinducing a vacuum of 75 cm Hg on the suction side.

[0071] The fibers used to form the skein may be formed of anyconventional membrane material provided the fibers are flexible and havean average pore cross sectional diameter for microfilitration, namely inthe range from about is 1000 Å to 10000 Å. Preferred fibers operate witha transmembrane pressure differential in the range from 7 kPa (1 psi)−69 kPa (10 psi) and are used under ambient pressure with the permeatewithdrawn under gravity. The fibers are chosen with a view to performtheir desired function, and the dimensions of the skein are determinedby the geometry of the headers ′ and length of the fibers. It isunnecessary to confine a skein in a modular shell, and a skein is not.

[0072] Preferred fibers are made of organic polymers and ceramics,whether isotropic, or anisotropic, with a thin layer or “skin” on theoutside surface of the fibers. Some fibers may be made from braidedcotton covered with a porous natural rubber latex or a water-insolublecellulosic polymeric material. Preferred organic polymers for fibers arepolysulfones, poly(styrenes), including styrene-containing copolymerssuch as acrylonitrile-styrene, butadiene-styrene andstyrene-vinylbenzylhalide copolymers, polycarbonates, cellulosicpolymers, polypropylene, poly(vinyl chloride), poly(ethyleneterephtbalate), and the like disclosed in U.S. Pat. No. 4,230,463 thedisclosure of which is incorporated by reference thereto as if fully setforth herein. Preferred ceramic fibers are made from alumina, by E. I.duPont deNemours Co. and disclosed in U.S. Pat. No. 4,069,157.

[0073] Typically, there is no cross flow of substrate across the surfaceof the fibers in a “dead end” tank. If there is any flow of substratethrough the skein in a dead end tank, the flow is due to aerationprovided beneath the skein, or to such mechanical mixing as may beemployed to maintain the solids in suspension. There is more flowthrough the skein in a tank into which substrate is being continuouslyflowed, but the velocity of fluid across the fibers is generally tooinsignificant to deter growing microorganisms from attaching themselves,or suspended particles, e.g. microscopic siliceous particles, from beingdeposited on the surfaces of the fibers.

[0074] For hollow fiber membranes, the outside diameter of a fiber is atleast 20 μm and may be as large as about 3 mm, typically being in therange from about 0.1 mm to 2 mm. The larger the outside diameter theless desirable the ratio of surface area per unit volume of fiber. Thewall thickness of a fiber is at least 5 μm and may be as much as 1.2 mm,typically being in the range from about 15% to about 60% of the outsidediameter of the fiber, most preferably from 0.5 mm to 1.2 mm.

[0075] As in a '424 array, but unlike in a conventional module, thelength of a fiber in a skein is essentially independent of the strengthof the fiber, or its diameter, because the skein is buoyed both bybubbles and the substrate in which it is deployed. The length of fibersin the skein is preferably determined by the conditions under which theskein is to operate. Typically fibers range from 1 m to about 5 m long,depending upon the dimensions of the body of substrate (depth and width)in which the skein is deployed.

[0076] The fixing material to fix the fibers in a finished header ismost preferably either a thermosetting or thermoplastic syntheticresinous material, optionally reinforced with glass fibers, boron orgraphite fibers and the like. Thermoplastic materials may becrystalline, such as polyolefins, polyamides (nylon), polycarbonates andthe like, semi-crystalline such as polyetherether ketone (PEEK), orsubstantially amorphous, such as poly(vinyl chloride) (PVC),polyurethane and the like. Thermosetting resins commonly includepolyesters, polyacetals, polyethers, cast acrylates, thermosettingpolyurethanes and epoxy resins. Most preferred as a “fixing” material(so termed because it fixes the locations of the fibers relative to eachother) is one which when cured is substantially rigid in a thickness ofabout 2 cm, and referred to generically as a “plastic” because of itshardness. Such a plastic has a hardness in the range from about Shore D50 to Rockwell R 110 and is selected from the group consisting of epoxyresins, phenolics, acrylics, polycarbonate, nylon, polystyrene,polypropylene and ultra-high molecular weight polyethylene (UHMW PE).Polyurethane such as is commercially available under the brand namesAdiprene® from Uniroyal Chemical Company and Airthane® from AirProducts, and commercially available epoxy resins such as Epon 828 areexcellent fixing materials.

[0077] The number of fibers in an array is arbitrary, typically being inthe range from about 1000 to about 10000 for commercial applications,and the preferred surface area for a skein is in the range from 10 m² to100 m². The particular method of securing the fibers in each of theheaders is not narrowly critical, the choice depending upon thematerials of the header and the fiber, and the cost of using a methodother than potting. However, it is essential that each of the fibers besecured in fluid-tight relationship within each header to avoidcontamination of permeate. This is effected by potting the fibersessentially vertically, in closely-spaced relationship, either linearlyin plural equally spaced apart rows across the face of a header in thex-y plane; or alternatively, randomly, in non-linear plural rows. In thelatter, the fibers are displaced relative to one another in the lateraldirection.

[0078]FIG. 1 presents the results of a comparison of three runs made,one using the teachings of Yamamoto in his '89 publication (curve 2),but using an aerator which introduced air from the side and directed itradially inwards, as is shown in Chiemchaisri et al. A second run (curve1) uses the gas-scrubbed assembly of the '424 patent, and the third run(curve 3) uses the gas-scrubbed skein of this invention. The specificflux obtained with an assembly of an inverted parabolic array with anair distributor means (Yamamoto et al), as disclosed in Wat. Sci. Tech.Vol. 21, Brighton pp 43-54, 1989, and, the parabolic array by Cote et alin the '424 patent, are compared to the specific flux obtained with thevertical skein of this invention.

[0079] The comparison is for the three assemblies having fibers withnominal pore size 0.2 μm with essentially identical bores and surfacearea in 80 L tanks filled with the same activated sludge substrate. Thedifferences between the stated experiment of Yamamoto et al, and that ofthe '424 patent are of record in the '424 patent, and the conditions ofthe comparison are incorporated by reference thereto as if fully setforth herein. The vertical skein used herein differs from the '424 skeinonly in the vertical configuration of the 280 fibers each of which wasabout 1% longer than the distance between the spaced apart headersduring operation. The flow rate of air for the vertical skein is 1.4m³/hr/m² using a coarse bubble diffuser.

[0080] It will be evident from FIG. 1 in which the specific flux,liters/meter² hr/kPa (conventionally written as (1 mh/kPa), is plottedas a function of operating time for the three assemblies, that thecurve, identified as reference numeral 3 for the flux for the verticalskein, provides about the same specific flux as the parabolic skein,identified as reference numeral 1. As can be seen, each specific fluxreaches an equilibrium condition within less than 50 hr, but after about250 hr, it is seen that the specific flux for the inverted parabolicarray keeps declining but the other two assemblies reach an equilibrium.

[0081] Referring to FIG. 2 there is illustrated, in exploded view aportion of a membrane device referred to as a “vertical skein” 10,comprising a lower header 11 of a pair of headers, the other upperheader (not shown) being substantially identical; a collection pan 20 tocollect the permeate; and, a permeate withdrawal conduit 30. The headershown is a rectangular prism since this is the most convenient shape tomake, if one is going to pot fibers 12 in a potting resin such as apolyurethane or an epoxy. Though the fibers 12 are not shown as closetogether as they would normally be, it is essential that the fibers arenot in contact with each other but that they be spaced apart by thecured resin between them.

[0082] As illustrated, the open ends of the terminal portion 12′ of thefibers are in the same plane as the lower face of the header 11 becausethe fibers are conventionally potted and the header sectioned to exposethe open ends. A specific potting procedure in which the trough of aU-shaped bundle of fibers is potted, results in forming two headers.This procedure is described in the '424 patent (col 17, lines 44-61);however, even cutting the potted fibers with a thin, high-speed diamondblade, tends to damage the fibers and initiate the collapse of thecircumferential wall. In another conventional method of potting fibers,described in U.S. Pat. No. 5,202,023, bundled fibers have their endsdipped in resin or paint to prevent potting resin penetration into thebores of the fibers during the potting process. The ends of the bundleare then placed in molds and uncured resin added to saturate the ends ofthe fiber bundle and fill the spaces between the individual fibers inthe bundle and the flexible tubing in which the bundle is held. Thecured molded ends are removed from the molds and the molded ends cut off(see, bridging cols 11 and 12). In each art method, sectioning the molddamages the embedded fibers.

[0083] Therefore a novel method is used to form a header 11 in the formof a rectangular prism. The method requires forming a composite headerwith two liquids. A first liquid fugitive material, when solidified(cured), forms a “fugitive lamina” of the composite header; a secondliquid of non-fugitive fixing material forms a “fixing lamina”. By a“fugitive material” we refer to a material which is either (i) solublein a medium in which the fibers and fixing material are not soluble, or(ii) fluidizable by virtue of having a melting point (if the material iscrystalline) below that which might damage the fibers or fixingmaterial; or, the material has a glass transition temperature Tg (if thematerial is non-crystalline), below that which might damage the fibersor material(s) forming the non-fugitive header; or (iii) both solubleand fluidizable.

[0084] The first liquid is poured around terminal portions of fibers,allowed to cool and solidify into a fugitive lamina; the fibers in thefugitive lamina are then again potted, this time by pouring the secondliquid over the solid fugitive lamina.

[0085] In greater detail, the method for forming a finished header forskein fibers comprises,

[0086] forming a stack of at least two superimposed essentially coplanarand similar arrays, each array comprising a chosen number of fiberssupported on a support means having a thickness corresponding to adesired lateral spacing between adjacent arrays;

[0087] holding the stack in a first liquid with terminal portions of thefibers submerged, until the liquid solidifies into a first shapedlamina, provided that the first liquid is unreactive with material ofthe fibers;

[0088] pouring a second liquid over the first shaped lamina to embed thefibers to a desired depth, and solidifying the second liquid to form afixing lamina upon the first shaped lamia, the second liquid also beingsubstantially unreactive with either the material of the fibers or thatof the first shaped lamina;

[0089] whereby a composite header is formed in which terminal portionsof the fibers are potted, preferably in a geometrically regular pattern,the composite header comprising a laminate of a fugitive lamina offugitive material and a contiguous finished header of fixing lamina; andthereafter,

[0090] removing the first shaped lamina without removing a portion ofthe fixing lamina so as to leave the ends of the fibers open andprotruding from the aft face of the header, the open ends havingcircular cross-section.

[0091] The step-wise procedure for forming an array “A” with the novelheader is described with respect to an array illustrated in FIG. 3, asfollows:

[0092] A desired number of fibers 12 are each cut to about the samelength with a sharp blade so as to leave both opposed ends of each fiberwith an essentially circular cross-section. The fibers are coplanarlydisposed side-by-side in a linear array on a planar support means suchas strips or cards 15 and 16. Preferably the strips are coated with anadhesive, e.g. a commercially available polyethylene hot-melt adhesive,so that the fibers are glued to the strips and opposed terminal portions12″ respectively of the fibers, extend beyond the strips. Intermediateportions 12′ of the fibers are thus secured on the strips.Alternatively, the strips may be grooved with parallel spaced-apartgrooves which snugly accommodate the fibers. The strips may be flexibleor rigid. If flexible, strips with fibers adhered thereto, are in turn,also adhered to each other successively so as to form a progressivelystiffer stack for a header having a desired geometry of potted fibers.To avoid gluing the strips, a regular pattern of linear rows may beobtained by securing multiple arrays on rigid strips in a stack, withrubber bands 18 or other clamping means. The terminal portions 12″ arethus held in spaced-apart relationship, with the center to centerdistance of adjacent fibers preferably in the range from 1.2 (1.2d) toabout 5 times (5d) the outside diameter ‘d’ of a fiber. Spacing thefibers further apart wastes space and spacing them closer increases therisk of fiber-to-fiber contact near the terminal end portions when theends are potted. Preferred center-to-center spacing is from about 1.5dto 2d. The thickness of a strip and/or adhesive is sufficient to ensurethat the fibers are kept spaced apart. Preferably, the thickness isabout the same as, or relatively smaller than the outside diameter of afiber, preferably from about 0.5d to 1d thick, which becomes the spacingbetween adjacent out-side surfaces of fibers in successive lineararrays.

[0093] Having formed a first array, a second array (not shown because itwould appear essentially identical to the first) is prepared in a manneranalogous to the first, strip 15 of the second array is overlaid uponthe intermediate portions 12′ on strip 15 of the first array, the strip15 of the second array resting on the upper surfaces of the fiberssecured in strip 15 of the first array. Similarly, strip 16 of thesecond array is overlaid upon the intermediate portions 12′ on strip 16of the first array.

[0094] A third array (essentially identical to the first and second) isprepared in a manner analogous to the first, and then overlaid upon thesecond, with the strips of the third array resting on the upper surfacesof the fibers of the second array.

[0095] Additional arrays are overlaid until the desired number of arraysare stacked in rows forming a stack of arrays with the adhesive-coatedstrips forming the spacing means between successive rows of fibers. Thestack of arrays on strips is then held vertically to present the lowerportion of the stack to be potted first.

[0096] Referring to FIG. 4, there is schematically illustrated arectangular potting pan 17 the length and width dimensions of whichcorrespond substantially to the longitudinal (x-axis) and transverse(y-axis) dimensions respectively, of the desired header. The lower stackis submerged in a first liquid which rises to a level indicated by L1,in the pan 17. Most preferred is a liquid wax, preferably awater-soluble wax having a melting point lower than 75° C., such as apolyetbylene glycol (PEG) wax.

[0097] The depth to which the first liquid is poured will depend uponwhether the strips 15 are to be removed from, or left in the finishedheader.

[0098] A. First illustrated is the potting of skein fibers in upper andlower headers from which the strips will be removed.

[0099] (1) A first shaped lamina having a thickness L1 (corresponding tothe-depth to which the first liquid was poured) is formed to provide afugitive lamina from about 5-10 cm thick. The depth of the first liquidis sufficient to ensure that both the intermediate portions 12′ on thestrips and terminal portions 12″ will be held spaced apart when thefirst liquid solidifies and plugs all the fibers.

[0100] (2) The second liquid, a curable, water-insoluble liquid pottingresin, or reactive components thereof, is poured over the surface of thefugitive lamina to surround the fibers, until the second liquid rises toa level L2. It is solidified to form the fixing lamina (which will bethe finished header) having a thickness measured from the level L1 tothe level L2 (the thickness is written “L1-L2”). The thickness L1-L2 ofthe fixing lamina, typically from about 1 cm to about 5 cm, issufficient to maintain the relative positions of the vertical fibers. Afirst composite header is thus formed having the combined thicknesses ofthe fugitive and fixing laminae.

[0101] (3) In a manner analogous to that described immediatelyhereinabove, a stack is potted in a second composite header.

[0102] (4) The composite headers are demolded from their potting pansand hot air blown over them to melt the fugitive laminae, leaving onlythe finished headers, each having a thickness L1-L2. The fugitivematerial such as the PEG wax, is then reused. Alternatively, awater-soluble fugitive material may be placed in hot water to dissolvethe wax, and the material recovered from its water solution.

[0103] (5) The adhered strips and terminal portions of the fibers whichwere embedded within the fugitive lamina are left protruding from thepermeate-discharging aft faces of the headers with the ends of thefibers being not only open, but essentially circular in cross section.The fibers may now be cut above the strips to discard them and theterminal portions of the fibers adhered to them, yet maintaining thecircular open ends. The packing density of fibers, that is, the numberof fibers per unit area of header preferably ranges from 4 to 50fibers/cm² depending upon the diameters of the fibers.

[0104] B. Illustrated second is the potting of skein fibers in upper andlower headers from which the strips will not be removed, to avoid thestep of cutting the fibers.

[0105] (1) The first liquid is poured to a level L1′ below the cards, toa depth in the range from about 1-2.5 cm, and solidified, formingfugitive lamina L1′.

[0106] (2) The second liquid is then poured over the fugitive lamina todepth 12 and solidified, forming a composite header with a fixing laminahaving a thickness L1′-L2.

[0107] (3) The composite header is demolded and the fugitive laminaremoved, leaving the terminal portions 12″ protruding from the aft faceof the finished header, which aft face is formed at what had been thelevel L1′. The finished header having a thickness L1′-L2 embeds thestrips 15 (along with the rubber bands 18, if used).

[0108] C. Illustrated third is the potting of skein fibers to form afinished headers with a cushioning lamina embedding the fibers on theopposed (fore) faces of the headers from which the strips will beremoved.

[0109] The restricted swayability of the fibers generates someintermittent ‘snapping’ motion of the fibers. This motion has been foundto break the potted fibers around their circumferences, at the interfaceof the fore face and substrate. The hardness of the fixing materialwhich forms a “fixing lamina” was found to initiate excessive shearingforces at the circumference of the fiber. The deleterious effects ofsuch forces is minimized by providing a cushioning lamina of materialsofter than the fixing lamina. Such a cushioning lamina is formedintegrally with the fixing lamina, by pouring cushioning liquid (sotermed for its function when cured) over the fixing lamina to a depth L3as shown in FIG. 4, which depth is sufficient to provide enough ‘give’around the circumferences of the fibers to minimize the risk ofshearing. Such cushioning liquid, when cured is rubbery, having ahardness in the range from about Shore A 30 to Shore D 45, and ispreferably a polyuretbane or silicone or other rubbery material whichwill adhere to the fixing lamina. Upon removal of the fugitive lamina,the finished header thus formed has the combined thicknesses of thefixing lamina and the cushioning lamina, namely L1-L3 when the strips 15are cut away.

[0110] D. Illustrated fourth is the formation a finished header with agasketing lamina embedding the fibers on the header's aft face, and acushioning lamina embedding the fibers on the header's fore face; thestrips are to be removed.

[0111] Whichever finished header is made, it is preferably fitted into apermeate pan 20 as illustrated in FIG. 2 with a peripheral gasket. Ithas been found that it is easier to seal the pan against a gasketinglamina, than against a peripheral narrow gasket. A relatively softgasketing material having a hardness in the range from Shore A 40 toShore D 45, is desirable to form a gasketing lamina integrally with theaft face of the finished header. In the embodiment in which the stripsare cut away, the fugitive lamina is formed as before, and a gasketingliquid (so termed because it forms the gasket when cured) is poured overthe surface of the fugitive lamina to a depth L4. The gasketing liquidis then cured. Upon removal of the fugitive lamina, when the strips 15are cut away, the finished header thus formed has the combinedthicknesses of the gasketing lamina (L1-L4), the fixing lamina (L4-L1)and the cushioning lamina (L2-L3), namely an overall L1-L3.

[0112] In another embodiment, to avoid securing the pan to the headerwith a gasketing means, and, to avoid positioning one or moregas-distribution manifolds in an optimum location near the base of theskein fibers after a skein is made, the manifolds are formed integrallywith a header. Referring to FIG. 5 there is illustrated in perspectiveview an “integral single skein” referred to generally by referencenumeral 100. The integral single skein is so termed because it includesan integral finished header 101 and permeate pan 102. The pan 102 isprovided with a permeate withdrawal nipple 106, and fitted with verticalair-tubes 103 which are to be embedded in the finished header. Theair-tubes are preferably manifolded on either side of the skein fibers,to feeder air-tubes 104 and 105 which are snugly inserted throughgrommets in the walls of the pan. The permeate nipple 106 is thenplugged, and a stack of arrays is held vertically in the pan in which afugitive lamina is formed embedding both the ends of the fibers and thelower portion of the vertical air-tubes 103. A fixing lamina is thenformed over the fugitive lamina, embedding the fibers to form a fixinglamina through which protrude the open ends of the air-tubes 103. Thefugitive lamina is then melted and withdrawn through the nipple 106. Inoperation, permeate collects in the permeate pan and is withdrawnthrough nipple 106.

[0113]FIG. 6 illustrates a cross-section of an integral single skein 110with another integral finished header 101 having a thickness L1-L2, butwithout a cushioning lamina, formed in a procedure similar to thatdescribed hereinabove. A permeate pan 120 with outwardly flared sides120′ and transversely spaced-apart through-apertures therein, isprefabricated between side walls 111 and 112 so the pan is spaced abovethe bottom of the reservoir.

[0114] A pair of air-manifolds 107 such as shown in FIG. 7A or 7B, ispositioned and held in mirror-image relationship with each otheradjacent the permeate pan 120, with the vertical air-tubes 103protruding through the apertures in sides 120′, and the ends 104 and 105protrude from through-passages in the vertical walls on either side ofthe permeate pan. Permeate withdrawal nipple 106 (FIG. 6) is firsttemporarily plugged. The stack of strips 15 is positioned betweenair-tubes 103, vertically in the pan 120 which is filled to level L1 toform a fugitive lamina, the level being just beneath the lower edges ofthe strips 15 which will not be removed. When solidified, the fugitivelamina embeds the terminal portions of the fibers 12 and also fillspermeate tube 106. Then the second liquid is poured over the uppersurface of the fugitive lamina until the liquid covers the strips 15 butleaves the upper ends of the air-tubes 103 open. The second liquid isthen cured to form the fixing lamina of the composite header which isthen heated to remove the fugitive material through the permeate nozzle106 after it is unplugged.

[0115]FIG. 7A schematically shows in perspective view, an air-manifold107 having vertical air-tubes 103 rising from a transverse header-tubewhich has longitudinally projecting feeder air-tubes 104 and 105. Thebore of the air-tubes which may be either “fine bubble diffusers”, or“coarse bubble diffusers”, or “aerators”, is chosen to provide bubblesof the desired diameter under operating conditions, the bore typicallybeing in the range from 0.1 mm to 5 mm. Bubbles of smaller diameter arepreferably provided with a perforated transverse tube 103′ of anair-manifold 107′ having feeder air-tubes 104′ and 105′, illustrated inFIG. 7B. In each case, the bubbles function as a mechanical brush.

[0116] The skein fibers for the upper header of the skein are potted ina manner analogous to that described above in a similar permeate pan toform a finished header, except that no air manifolds are inserted.

[0117] Referring to FIG. 8 there is schematically illustrated, in across-sectional perspective view, an embodiment in which a bank of twoskeins is potted in a single integral finished header enclosure,referred to generally by reference numeral 120 b. The term “headerenclosure” is used because its side walls 121 and 122, and end walls(not shown) enclose a plenum in which air is introduced. Instead of apermeate pan, permeate is collected from a permeate manifold whichserves both skeins. Another similar upper enclosure 120 u (not shown),except that it is a flat-bottomed channel-shaped pan (inverted for useas the upper header) with no air-tubes molded in it, has the opposedterminal portions of all the skein fibers potted in the pan. Foroperation, both the lower and upper enclosures 120 b and 120 u, withtheir skein fibers are lowered into a reservoir of the substrate to befiltered. The side walls 121 and 122 need not rest on the bottom of thereservoir, but may be mounted on a side wall of the reservoir.

[0118] The side walls 121 and 122 and end walls are part of anintegrally molded assembly having a platform 123 connecting the walls,and there are aligned multiple risers 124 molded into the platform. Therisers resemble an inverted test-tube, the diameter of which need onlybe large enough to have an air-tube 127 inserted through the top 125 ofthe inverted test-tube. As illustrated, it is preferred to have “n+1”rows of air-tubes for “n” stacks of arrays to be potted. Crenelatedplatform 123 includes risers 124 between which lie channels 128 and 129.Channels 128 and 129 are each wide enough to accept a stack of arrays offibers 12, and the risers are wide enough to have air-tubes 127 ofsufficient length inserted therethrough so that the upper open ends 133of the air-tubes protrude from the upper surface of the fixing material101. The lower ends 134 of the air-tubes are sectioned at an angle tominimize plugging, and positioned above the surface S of the substrate.The channel 129 is formed so as to provide a permeate withdrawal tube126 integrally formed with the platform 123. Side wall 122 is providedwith an air-nipple 130 through which air is introduced into the plenumformed by the walls of the enclosure 120 b, and the surface S ofsubstrate under the platform 123. Each stack is potted as described inrelation to FIG. 6 above, most preferably by forming a composite headerof fugitive PEG wax and epoxy resin around the stacks of arrayspositioned between the rows of risers 124, making sure the open ends ofthe air-tubes are above the epoxy fixing material, and melting out thewax through the permeate withdrawal tube 126. When air is introducedinto the enclosure the air will be distributed through the air-tubesbetween and around the skeins.

[0119] Referring to FIG. 9 there is shown a schematic illustration of askein having upper and lower headers 41 u and 41 b respectively, and ineach, the protruding upper and lower ends 12 u″ and 12 b″ are evidencethat the face of the header was not cut to expose the fibers. The heightof the contiguous intermediate portions 12 u′ and 12 b′ respectively,corresponds to the cured depth of the facing material.

[0120] It will now be evident that the essential feature of theforegoing potting method is that a fugitive lamina is formed whichembeds the openings of the terminal portions of the fibers before theircontiguous intermediate portions 12 u′ and 12 u″ and 12 b′ and 12 b″respectively are fixed in a fixing lamina of the header. An alternativechoice of materials is the use of a fugitive potting compound which issoluble in a non-aqueous liquid in which the fixing material is notsoluble. Still another choice is to use a water-insoluble fugitivematerial which is also insoluble in non-aqueous liquids typically usedas solvents, but which fugitive material has a lower melting point thanthe final potting material which may or may not be water-soluble.

[0121] The fugitive material is inert relative to both, the material ofthe fibers as well as the final potting material to be cast, and thefugitive material and fixing material are mutually insoluble. Preferablythe fugitive material forms a substantially smooth-surfaced solid, butit is critical that the fugitive material be at least partially cured,sufficiently to maintain the shape of the header, and remain a solidabove a temperature at which the fixing material is introduced into theheader mold. The fugitive lamina is essentially inert and insoluble inthe final potting material, so that the fugitive lamina is removablyadhered to the fixing lamina.

[0122] The demolded header is either heated or solvent extracted toremove the fugitive lamina. Typically, the fixing material is cured to afirm solid mass at a first curing temperature no higher than the meltingpoint or Tg of the fugitive lamina, and preferably at a temperaturelower than about 60° C.; the firm solid is then post-cured at atemperature high enough to melt the fugitive material but not highenough to adversely affect the curing of the fixing material or theproperties of the fibers. The fugitive material is removed as describedhereinafter, the method of removal depending upon the fugitive materialand the curing temperature of the final potting material used.

[0123] Since, during operation, a high flux is normally maintained ifcleansing air contacts substantially all the fibers, it will be evidentthat when it is desirable to have a skein having a cross-section whichis other than generally rectangular, for example elliptical or circular,or having a geometrically irregular periphery, and it is desired to havea large number of skein fibers, it will be evident that the procedurefor stacking consecutive peripheral arrays described above will bemodified. Further, the transverse central air-tube 52 (see FIG. 9) isfound to be less effective in skeins of non-rectangular cross-sectionthan a vertical air-tube which discharges air radially along itsvertical length and which vertical air-tube concurrently serves as thespacing means. Such skeins with a generally circular or ellipticalcross-section with vertical air-tubes are less preferred to form a bankbut provide a more efficient use of available space in a reservoir thana rectangular skein.

[0124] Referring further to FIG. 2, the header 11 has front and rearwalls defined by vertical (z-axis) edges 11′ and longitudinal (x-axis)edges 13′; side walls defined by edges 11′ and transverse (y-axis) edges13″; and a base 13 defined by edges 13′ and 13″.

[0125] The collection pan 20 is sized to snugly accommodate the base 13above a permeate collection zone within the pan. This is convenientlydone by forming a rectangular pan having a base 23 of substantially thesame length and width dimensions as the base 13. The periphery of thepan 20 is provided with a peripheral step as shown in FIG. 2A, in whichthe wall 20′ of the pan terminates in a step section 22, having asubstantially horizontal shoulder 22″ and a vertical retaining wall 22′.

[0126]FIG. 2B is a bottom plan view of the lower face of header 13showing the open ends of the fibers 12′ prevented from touching eachother by potting resin. The geometrical distribution of fibers providesa regular peripheral boundary 14 (shown in dotted outline) which boundsthe peripheries of the open ends of the outermost fibers.

[0127] Permeate flows from the open ends of the fibers onto the base 23of the pan 20, and flows out of the collection zone through a permeatewithdrawal conduit 30 which may be placed in the bottom of the pan inopen flow communication with the inner portion of the pan. When theskein is backwashed, back-washing fluid flows through the fibers andinto the substrate. If desired, the withdrawal conduit may be positionedin the side of the pan as illustrated by conduit 30′. Whether operatingunder gravity alone, or with a pump to provide additional suction, itwill be apparent that a fluid-tight seal is necessary between theperiphery of the header 11 and the peripheral step 22 of the pan 20.Such a seal is obtained by using any conventional means such as asuitable sealing gasket or sealing compound, typically a polyurethane orsilicone resin, between the lower periphery of the header 11 and thestep 22. As illustrated in FIG. 2, permeate drains downward, but itcould also be withdrawn from upper permeate port 45 u in the upperpermeate pan 43 u (see FIG. 9).

[0128] It will now be evident that a header with a circular peripherymay be constructed, if desired. Headers with geometries having stillother peripheries (for example, an ellipse) may be constructed in ananalogous manner, if desired, but rectangular headers are most preferredfor ease of construction with multiple linear arrays.

[0129] Referring to FIGS. 9 and 2A, six rows of fibers 12 are shown oneither side of a gas distribution line 52 which traverses the length ofthe rows along the base of the fibers. The potted terminal end portions12 b″ open into permeate pan 43 b. Because portions 12 u′ and 12 b′ ofindividual fibers 12 are potted, and the fibers 12 are preferably from1% to 2% longer than the fixed distance between upper and lower headers41 u and 41 b, the fibers between opposed headers are generally parallelto one another, but are particularly parallel near each header. Alsoheld parallel are the terminal end portions 12 u″ and 12 b″ of thefibers which protrude from the headers with their open ends exposed. Thefibers protrude below the lower face of the bottom header 41 b, andabove the upper face of the upper header 41 u. The choice of fiberspacing in the header will determine packing density of the fibers nearthe headers, but fiber spacing is not a substantial considerationbecause spacing does not substantially affect specific flux duringoperation. It will be evident however, that the more fibers, the moretightly packed they will be, giving more surface area.

[0130] Since the length of fibers tends to change while in service, theextent of the change depending upon the particular composition of thefibers, and the spacing between the upper and lower headers is critical,it is desirable to mount the headers so that one is adjustable in thevertical direction relative to the other, as indicated by the arrow V.This is conveniently done by attaching the pan 43 u to a plate 19 havingvertically spaced apart through-passages 34 through which a threadedstud 35 is inserted and secured with a nut 36. Threaded stud 35 is in afixed mounting block 37.

[0131] The density of fibers in a header is preferably chosen to providethe maximum membrane surface area per unit volume of substrate withoutadversely affecting the circulation of substrate through the skein. Agas-distribution means 52 such as a perforated air-tube, provides airwithin the skein so that bubbles of gas (air) rise upwards whileclinging to the outer surfaces of the fibers, thus efficiently scrubbingthem. If desired, additional air-tubes 52′ may be placed on either sideof the skein near the lower header 41 b, as illustrated in phantomoutline, to provide additional air-scrubbing power. Whether the permeateis withdrawn from the upper header through port 45 u or the lower headerthrough port 45 b, or both, depends upon the particular application, butin all instances, the fibers have a substantially vertical orientation.

[0132] The vertical skein is deployed in a substrate to present agenerally vertical profile, but has no structural shape. Such shape asit does have changes continuously, the degree of change depending uponthe flexibility of the fibers, their lengths, the overall dimensions ofthe skein, and the degree of movement imparted to the fibers by thesubstrate and also by the oxygen-containing gas from thegas-distribution means.

[0133] Referring to FIG. 10 there is illustrated a typical assemblyreferred to as a “wall-mounted bank” which includes at least twoside-by-side skeins, indicated generally by reference numerals 40 and40′ with their fibers 42 and 42′; fibers 42 are potted in upper andlower headers 41 u and 41 b respectively; and fibers 42′ in headers 41u′ and 41 b′; headers 41 u and 41 b are fitted with permeate collectingmeans 46 u and 46 b respectively; headers 41 u′ and 41 b′ are fittedwith permeate collecting means 46 u′ and 46 b′ respectively; and, theskeins share a common gas-distribution means 50. A “bank” of skeins istypically used to retrofit a large, deep tank from which permeate is tobe withdrawn using a vacuum pump. In a large reservoir, several banks ofskeins may be used in side-by-side relationship within a tank. Eachskein includes multiple rows (only one row is shown) of fibers 42 and42′ in upper headers 41 u and 41 u′, and lower headers 41 b and 41 b′respectively, and arms 51 and 51′ of gas-distribution means 50 aredisposed between the lower headers 41 b and 41 b′, near their bases. Theupper headers 44 u and 44 u′ are mounted by one of their ends to avertical interior surface of the wall W of a tank, with mountingbrackets 53 and 53′ and suitable fastening means such as bolts 54. Thewall W thus functions as a spacer means which fixes the distance betweenthe upper and lower headers. Each upper header is provided with apermeate collection pan 43 u and 43 u′, respectively, connected topermeate withdrawal conduits 45 u and 45 u′ and manifolded to permeatemanifold 46 u through which permeate being filtered into the collectionpans is continuously withdrawn. Each header is sealingly bonded aroundits periphery, to the periphery of each collection pan.

[0134] The skein fibers (only one array of which is shown for clarity)shown in this perspective view have an elongated rectangularparallelpiped shape the sides of which are irregularly shaped whenimmersed in a substrate, because of the random side-to-side displacementof fibers as they sway. An elongated rectangular parallelpiped shape ispreferred since it permits a dense packing of fibers, yet results inexcellent scrubbing of the surfaces of the fibers with bubbles. Withthis shape, a skein may be formed with from 10 to 50 arrays of fibersacross the longitudinal width ‘w’ of the headers 41 u, 41 b, and 41 u′,41 b′ with each array having fibers extending along the transverselength ‘l’ of each header. Air-tubes on either side of a skeineffectively cleanse the fibers if there are less than about 30 arraysbetween the air-tubes. A skein having more than 30 arrays is preferablyalso centrally aerated as illustrated by the air-tube 52 in FIG. 9.

[0135] Thus, if there are about 100 fibers closely spaced-apart alongthe transverse length ‘l’ of an array, and there are 25 arrays in askein in a header of longitudinal width ‘w’, then the opposed terminalend portions of 2500 fibers are potted in headers 41 u and 41 b. Theopen ends of all fibers in headers 41 b and 41 b′ point downwards intocollection zones in collection pans 43 b and 43 b′ respectively, andthose of all fibers in headers 41 u and 41 u′ point upwards intocollection zones in collection pans 43 u and 43 u′ respectively.Withdrawal conduits 45 u and 45 u′ are manifolded to permeate manifold46 u through which permeate collecting in the upper collection pans 43 uand 43 u′ is typically continuously withdrawn. If the permeate flow ishigh enough, it may also be withdrawn from the collection pans 43 b and43 b′ through withdrawal conduits 45 b and 45 b′ which are manifolded topermeate manifold 46 b. When permeate is withdrawn in the same plane asthe permeate withdrawal conduits 45 u, 45 u′ and manifold 46 u, and thetransmembrane pressure differential of the fibers is in the range from35-75 kPa (5-10 psi), manifold 46 u may be connected to the suction sideof a centrifugal pump which will provide adequate NPSH.

[0136] In general, the permeate is withdrawn from both the upper andlower headers, until the flux declines to so low a level as to requirethat the fibers be backwashed. The skeins may be backwashed byintroducing a backwashing fluid through the upper permeate collectionmanifold 46 u, and removing the fluid through the lower manifold 46 b.Typically, from 3 to 30 skeins may be coupled together for internalfluid communication with one and another through the headers, permeatewithdrawal means and the fibers; and, for external fluid communicationwith one another through an air manifold. Since the permeate withdrawalmeans is also used for backflushing it is generally referred to as a‘liquid circulation means’, and as a permeate withdrawal means only whenit is used to withdraw permeate.

[0137] When deployed in a substrate containing suspended and dissolvedorganic and inorganic matter, most fibers of organic polymers remainbuoyant in a vertical position. The fibers in the skein are floatinglybuoyed in the substrate with the ends of the fibers anchored in theheaders. This is because (i) the permeate is essentially pure waterwhich has a specific gravity less than that of the substrate, and mostpolymers from which the fibers are formed also have a specific gravityless than 1, and, (ii) the fibers are buoyed by bubbles which contactthem. Fibers made from ceramic, or, glass fibers are heavier than water.

[0138] Adjacent the skeins, an air-distribution manifold 50 is disposedbelow the base of the bundle of fibers, preferably below the horizontalplane through the horizontal center-lines of the headers. The manifold50 is preferably split into two foraminous arms 51 and 51′ adjacent thebases of headers 41 b and 41 b′ respectively, so that when air isdischarged through holes in each portion 51 and 51′, columns of bubblesrise adjacent the ends of the fibers and thereafter flow along thefibers through the skeins. If desired, additional portions (not shown)may be used adjacent the bases of the lower headers but located on theoutside of each, so as to provide additional columns of air along theouter surfaces of the fibers.

[0139] The type of gas (air) manifold is not narrowly critical providedit delivers bubbles in a preferred size range from about 1 nm to 25 mm,measured within a distance of from 1 cm to 50 cm from thethrough-passages generating them. If desired, each portion 51 and 51′may be embedded in the upper surface of each header, and the fiberspotted around them, making sure the air-passages in the portions 51 and51′ are not plugged with potting compound. If desired, additional armsof air-tubes may be disposed on each side of each lower header, so thatfibers from each header are scrubbed by columns of air rising fromeither transverse side.

[0140] The air may be provided continuously or intermittently, betterresults generally being obtained with continuous air flow. The amount ofair provided depends upon the type of substrate, the requirements of thetype of microorganisms, if any, and the susceptibility of the surfacesof the fibers to be plugged, there always being sufficient air toproduce desired growth of the microorganisms when operated in asubstrate where maintaining such growth is essential.

[0141] Referring to FIG. 11, there is schematically illustrated anotherembodiment of an assembly, referred to as a “stand-alone bank” ofskeins, two of which are referenced by numeral 60. The bank is referredto as being a “stand-alone” because the spacer means between headers issupplied with the skeins, usually because mounting the skeins againstthe wall of a reservoir is less effective than placing the bank inspaced-apart relationship from a wall. In other respects, the bank 60 isanalogous to the wall-mounted bank illustrated in FIG. 10.

[0142] Each bank 60 with fibers 62 (only a single row of the multiple,regularly spaced apart generally vertical arrays is shown for the sakeof clarity) is deployed between upper and lower headers 61 u and 61 b ina substrate ‘S’. The lower headers rest on the floor of the reservoir.The upper headers are secured to rigid vertical air tubes 71 and 71′through which air is introduced into a tubular air manifold identifiedgenerally by reference numeral 70. The manifold 70 includes (i) thevertical tubular arms 71 and 71′; (ii) a lower transverse arm 72 whichis perforated along the length of the lower header 61 b′ and securedthere-to; the arm 72 communicates with longitudinal tubular arm 73, andoptionally another longitudinal arm 73′ (not shown) in mirror-imagerelationship with arm 73 on the far side of the headers; and (iii)transverse arms 74 and 74′ in open communication with 72 and 73; arms 74and 74′ are perforated along the visible transverse faces of the headers61 b an 61 b′, and 74 and 74′ may communicate with tubular arm 73′ if itis provided. The vertical air-tubes 71 and 71′ conveniently provide theadditional function of a spacer means between the first upper header andthe first lower header, and because the remaining headers in the bankare also similarly (not shown) interconnected by rigid conduits, theheaders are maintained in vertically and transversely spaced-apartrelationship. Since all arms of the air manifold are in opencommunication with the air supply, it is evident that uniformdistribution of air is facilitated.

[0143] As before, headers 61 u and 61 u′ are each secured in fluid-tightrelationship with collection zones in collection pans 63 u and 63 u′respectively, and each pan has withdrawal conduits 65 u and 65 u′ whichare manifolded to longitudinal liquid conduits 81 and 81′. Analogously,headers 61 b and 61 b′ are each secured in fluid-tight relationship withcollection zones in collection pans 63 b and 63 b′ respectively, andeach pan has withdrawal conduits 65 b and 65 b′ which are manifolded tolongitudinal conduits 82 and 82′. As illustrated, withdrawal conduitsare shown for both the upper and the lower headers, and both fore andaft the headers. In many instances, permeate is withdrawn from only anupper manifold which is provided on only one side of the upper headers.A lower manifold is provided for backwashing. Backwashing fluid istypically flowed through the upper manifold, through the fibers and intothe lower manifold. The additional manifolds on the aft ends of theupper and lower headers not only provides more uniform distribution ofbackwashing fluid but support for the interconnected headers. It will beevident that, absent the aft interconnecting upper conduit 81′, an upperheader such as 61 u will require to be spaced from its lower header bysome other interconnection to header 61 u′ or by a spacer strut betweenheaders 61 u and 61 b.

[0144] In the best mode illustrated, each upper header is provided withrigid PVC tubular nipples adapted to be coupled with fittings such asells and tees to the upper conduits 81 and 81′ respectively.Analogously, each lower header is connected to lower conduits 82 and 82′(not shown) and/or spacer struts are provided to provide additionalrigidity, depending upon the number of headers to be interconnected.Permeate is withdrawn through an upper conduit, and all pipingconnections, including the air connection are made above the liquidlevel in the reservoir.

[0145] The length of fibers (between headers) in a skein is generallychosen to obtain efficient use of an economical amount of air, so as tomaintain optimum flux over a long period of time. Other considerationsinclude the depth of the tank in which the bank is to be deployed, thepositioning of the liquid and air manifolds, and the convection patternswithin the tank, inter alia.

[0146] In another embodiment of the invention, a bioreactor isretrofitted with plural banks of skeins schematically illustrated in theelevational view shown in FIG. 12, and the plan view shown in FIG. 13.The clarifier tank is a large circular tank 90 provided with a vertical,circular outer baffle 91, a vertical circular inner baffle 92, and abottom 93 which slopes towards the center (apex) for drainage ofaccumulating sludge. Alternatively, the baffles may be individual,closely spaced rectangular plates arranged in outer and inner circles,but continuous cylindrical baffles (shown) are preferred. Irrespectiveof which baffles are used, the baffles are located so that their bottomperipheries are located at a chosen vertical distance above the bottom.Feed is introduced through feed line 94 in the bottom of the tank 90until the level of the substrate rises above the outer baffle 91.

[0147] A bank 60 of plural skeins 10, analogous to those in the bankdepicted in FIG. 10, each of which skeins is illustrated in FIG. 9, isdeployed against the periphery of the inner wall of the bioreactor withsuitable mounting means in an outer annular permeate extraction zone 95′(FIG. 13) formed between the circular outer baffle 91 and the wall ofthe tank 90, at a depth sufficient to submerge the fibers. Aclarification zone 91′ is defined between the outer circular baffle 91and inner circular baffle 92. The inner circular baffle 92 provides avertical axial passage 92′ through which substrate is fed into the tank90. The skeins form a dense curtain of fibers in radially extending,generally planar vertical arrays as illustrated in FIG. 9, pottedbetween upper and lower headers 41 u and 41 b. Permeate is withdrawnthrough manifold 46 u and air is introduced through air-manifold 80,extending along the inner wall of the tank, and branching out withair-distribution arms between adjacent headers, including outerdistribution arms 84′ on either side of each lower header 41 b at eachend of the bank. The air manifold 80 is positioned between skeins in thepermeate extraction zone 95′ in such a manner as to have bubbles contactessentially the entire surface of each fiber which is continuously awashwith bubbles. Because the fibers are generally vertical, the air is incontact with the surfaces of the fibers longer than if they werearcuate, and the air is used most effectively to maintain a high fluxfor a longer period of time than would otherwise be maintained.

[0148] It will be evident that if the tank is at ground level, therewill be insufficient liquid head to induce a desirable liquid head undergravity alone. Without an adequate siphoning effect, a centrifugal pumpmay be used to produce the necessary suction. Such a pump should becapable of running dry for a short period, and of maintaining a vacuumon the suction side of from cm (10″) −51 cm (20″) of Hg, or −35 kPa (−5psi) to −70 kPa (−10 psi). Examples of such pumps rated at 18.9 L/min (5gpm)@15″ Hg, are (i) flexible impeller centrifugal pumps, e.g. Jabsco#30510-2003; (ii) air operated diaphragm pumps, e.g. Wilden M2; (iii)progressing cavity pumps, e.g. Ramoy 3561; and (iv) hosepumps, e.g.Waukesha SP 25.

[0149] The skein may also be potted in a header which is not arectangular prism, preferably in cylindrical upper and lower headers inwhich substantially concentric arrays of fibers are non-removably pottedin cylindrical permeate pans, and the headers are spaced apart by acentral gas tube which functions as both the spacer means and thegas-distribution means which is also potted in the headers. As before,the fibers are restrictedly swayable, but permeate is withdrawn fromboth upper and lower headers through a single permeate pan so that allconnections for the skein, when it is vertically submerged, are fromabove. Permeate is preferably withdrawn from the lower permeate panthrough a central permeate withdrawal tube which is centrally axiallyheld within the central gas (air) tube. The concentric arrays are formedby wrapping successive sheets of flat arrays around the centralair-tube, and gluing them together before they are potted. Thisconfiguration permits the use of more filtration surface area per unitvolume of a reservoir, compared to skeins with rectangular prismheaders, using the same diameter and length of fibers.

[0150] FIGS. 14-17 specifically illustrate preferred embodiments of thecylindrical vertical skein. Referring to FIG. 14 there is schematicallyillustrated, in cross-sectional elevational view a vertical cylindricalskein 210 resting on the floor F of a tank, the skein comprising a pairof similar upper and lower cylindrical end-caps 221 and 222respectively, which serve as permeate collection pans. Bores 226 and 227in the upper and lower end-caps have permeate withdrawal tubes 231 and232, respectively, connected in fluid-tight engagement therein. Permeatewithdrawn through the tubes is combined in a permeate withdrawalmanifold 230. Each end-cap has a finished upper/lower header formeddirectly in it, upper header 223 being substantially identical to lowerheader 224. Each header is formed by potting fibers 212 in a pottingresin such as a polyurethane or an epoxy of sufficient stiffness to holdand seal the fibers under the conditions of use. A commerciallyavailable end-cap for poly (vinyl chloride) “PV C” pipe is mostpreferred; for large surface area skeins, larger headers are provided bycommercially available glass fiber reinforced end-caps for cylindricaltanks. It is essential that the fibers are not in contact with eachother, but spaced apart by cured resin. It is also essential that thecured resin adhere to and seal the lower portions 212′ of each of thefibers against leakage of fluid under operating conditions of the skein.Visual confirmation of a seal is afforded by the peripheries of thefibers being sealed at the upper (fore) and lower (aft) faces 223 u and223 b respectively of the upper header 223, and the fore and aft faces224 u and 224 b respectively of the lower header 224. A conventionalfinished header may be used in which the ends 212″ of the fibers wouldbe flush (in substantially the same plane) as the lower face 224 b. Inthe best mode, though not visible through an opaque end-cap, the openends 212″ of the fibers protrude from the headers' lower (aft or bottom)face 224 b.

[0151] The finished upper header 223 (fixing lamina) is left adhered tothe periphery of the end-cap 221 when the fugitive lamina is removedthrough bore 226 in the upper header; and analogously, the finishedlower header 224 is left adhered to the periphery of the end-cap 222when the fugitive lamina is removed through a bore 227.

[0152] Skein fibers 212 are preferably in arrays bundled in a geometricconfiguration such as a spiral roll. A header is formed in a manneranalogous to that described in relation to FIG. 4, by potting the lowerend of the spiral roll. FIG. 14A, showing a bottom plan view of the aftface 224 b of header 224, illustrates the spiral pattern of openings inthe ends 212″ of the fibers. It is preferred, before an array is rolledinto a spiral, to place a sparger 240 (shown in FIG. 15A) with a rigidair-supply tube 242 in the array so that upon farming a spiral roll theair-supply tube is centrally axially held within the roll.

[0153] Illustrated in FIG. 14B is a bottom plan view of aft face 224 bwith another configuration, wherein a series of successively largerdiameter circular arrays are formed, each a small predetermined amountlarger than the preceding one, and the arrays secured, preferablyadhesively, one to the next, near their upper and lower peripheriesrespectively to form a dense cylindrical mass of fibers. In such a massof fibers, referred to as a series of annular rings, each array issecured both to a contiguous array having a next smaller diameter, aswell as to a contiguous array having a next larger diameter, except forthe innermost and outermost arrays which have the smallest and largestdiameters, respectively. The pattern in header 224 illustrates the ends212″ of the fibers after the nested arrays are potted.

[0154] Illustrated in FIG. 14C is a bottom plan view of lower (aft) face224 b with plural arrays arranged chord-like within the header 224.Arrays are formed on pairs of strips, each having a length correspondingto its position as a chord within a potting ring in which the skeinfibers are to be potted. That is, each array is formed on strips ofdiminishing width, measured from the central array which is formed on astrip having a width slightly less than the inner diameter of thepotting ring in which the stack is to be potted. The arrays are stackedwithin the ring, the widest array corresponding in position to thediameter of the ring. For a chosen fiber 212, the larger the surfacearea required in a skein, the greater the number of fibers in eacharray, the bigger the diameter of the ring, and the wider eachchord-like array. The plural arrays are preferably adhered one to theother by coating the surfaces of fibers with adhesive prior to placing astrip of the successive array on the fibers. Alternatively, the bundledarrays may be held with a rubber band before being inserted in thepotting ring. The resulting chord-like pattern in header 224 illustratesthe ends 212″ of the fibers after the nested arrays are potted.

[0155] A detail of a sparger 240 is provided in FIG. 15A. Thestar-shaped sparger 240 having radially outwardly extending tubular arms241 and a central supply stub 242, supplies air which is directed intothe tubular arms and discharged into the substrate through air passagesin the walls of the arms. An air feed tube 244 connected to the centralsupply stub 242 provides air to the sparger 240. The lower end of thecentral stub 242 is provided with short projecting nipples 245 the innerends of which are brazed to the stub. The outer ends of the nipples arethreaded. The central stub and nipples are easy to insert into thecenter of the mass of skein fibers. When centrally positioned, arms 241which are threaded at one end, are threadedly secured to the outer endsof the nipples.

[0156] As illustrated in FIG. 14, lower end-cap 222 rests on the floor Fof a tank, near a vertical wall W to which is secured a verticalmounting strut 252 with appropriate fastening means such as a nut 253and bolt 254. A U-shaped bracket 251 extends laterally from the base ofthe mounting strut 252. The arms of the U-shaped bracket support theperiphery of upper end-cap 221, and to ensure that the end-cap stays inposition, it is secured to the U-shaped bracket with a right anglebracket and fastening means (not shown). A slot in mounting strut 252permits the U-shaped bracket to be raised or lowered so that the desireddistance between the opposed faces 223 b and 224 u of the upper andlower headers respectively is less than the length of any potted fiber,measured between those faces, by a desired amount. Adjustability isparticularly desirable if the length of the fibers tends to changeduring service.

[0157] As illustrated in FIG. 14, if it is desirable to withdrawpermeate from only the upper tube 231, a permeate connector tube 233(shown in phantom outline), is inserted within the mass of skein fibers212 through the headers 223 and 224, connecting the permeate collectionzone. 229 in the lower end-cap in open fluid communication with thepermeate collection zone 228 in the upper end-cap; and, bore 227 isplugged with a plug 225 as shown in FIG. 15. Since, under suchcircumstances, it does not matter if the lower ends 212″ of the fibersare plugged, and permeate collection zone 229 serves no essentialfunction, the zone 229 may be filled with potting resin.

[0158] Referring to FIG. 16 there is illustrated a skein 270 with upperand lower end-caps in which are sealed upper and lower ring headersformed in upper and lower rings 220 u and 220 b respectively, after thefibers in the skein are tested to determine if any is defective. A rigidair-supply tube 245 is positioned in the spiral roll as described above,and the lower end of the roll is potted forming a lower finished header274 in which the lower end 246 of the air-supply tube is potted, fixingthe position of the arms 241 of the sparger just above the upper face274 u of the header 274.

[0159] In an analogous manner, an upper header 273 is formed in ring 220u and upper end 247 of air-supply tube 245 is inserted through an axialbore 248 with-in upper end-cap 271 which is slipped over the ring 220 uthe outer periphery of which is coated with a suitable adhesive, to sealthe ring 220 u in the end-cap 271. The periphery of the upper end 247 issealed in the end cap 271 with any conventional sealing compound.

[0160] Referring to FIG. 17 there is schematically illustrated anotherembodiment of a skein 280 in which rigid permeate tube 285 is heldconcentrically within a rigid air-supply tube 286 which is pottedaxially within skein fibers 212 held between opposed upper and lowerheaders 283 and 284 in upper and lower rings 220 u and 220 b which arein turn sealed in end-caps 281 and 282 respectively. For ease ofmanufacture, the lower end 285 b of permeate tube 285 is snugly fittedand sealed in a bushing 287. The bushing 287 and end 285 b are theninserted in the lower end 286 b of the air supply tube 286 and sealed init so that the annular zone between the outer surface of permeate tube285 and the inner surface of air supply tube 286 will duct air to thebase of the fibers but not permit permeate to enter the annular zone.The air supply tube is then placed on an array and the array is rolledinto a spiral which is held at each end with rubber bands. The lower endof the roll is placed in a ring 220 b and a lower ring header is formedwith a finished header 284 as described above It is preferred to use arelatively stiff elastomer having a hardness in the range from 50 ShoreA to about 20 Shore D, and most preferred to use a polyurethane having ahardness in the range from 50 Shore A to about 20 Shore D, measured asset forth in ASTM D-790, such as PTU-921 available from CanadianPoly-Tech Systems. To form the upper finished header 283 the air supplytube is snugly inserted through an O-ring held in a central bore in aplate such as used in FIG. 5, to avoid loss of potting resin from thering, and the fugitive resin and finishing resins poured and cured,first one then the other, in the ring. Lower finished header 284 isformed with intermediate portions 212 b′ embedded, and terminal portions212 b″ protruding from the header's aft face. Upper finished header 283is formed with intermediate portions 212 u′ embedded, and terminalportion 212 u′ protruding from the header's fore face. After thefinished headers 283 and 284 are formed and the fibers checked fordefects, the upper end 286 uof the air supply tube 286 is insertedthrough a central bore 288 in upper end-cap 281 and sealed within thebore with sealing compound or a collar 289.

[0161] Preferably the permeate tube 285, the air supply tube 286 and thecollar 289 are all made of PVC so that they are easily cemented togetherto make leak-proof connections.

[0162] As shown, permeate may be withdrawn through the permeate tube 285from the permeate collection zone in the lower end-cap 282, andseparately from the upper end-cap 281 through permeate withdrawal port281 p which may be threaded for attaching a pipe fitting. Alternatively,the permeate port 281 p may be plugged and permeate withdrawn from bothend-caps through the permeate tube 285.

[0163] Upper end 285 u and upper end 286 u of air supply tube 286 areinserted through a T-fitting 201 through which air is supplied to theair supply tube 286. The lower end 201 b of one of the arms of the T 201is slip-fitted and sealed around the air supply tube. The upper end 201u of the other arm is inserted in a reducing bushing 202 and sealedaround the permeate tube. Air supplied to intake 203 of the T 201travels down the annular zone between the permeate tube and the airsupply tube and exits through opposed ports 204 in the lower portion ofthe air supply tube, just above the upper face 284 u of the lower header284. It is preferred to thread ports 204 to threadedly secure the endsof arms 241 to form a sparger which distributes air substantiallyuniformly across and above the surface 284 u. Additional ports may beprovided along the length of the vertical air supply tube, if desired.

EXAMPLE 1

[0164] Microfiltration of an activated sludge at 30° C. having aconcentration of 25 g/L (2.5% TSS) is carried out with a skein ofpolysulfone fibers in a pilot plant tank. The fibers are “air scrubbed”at a flow rate of 12 CFM (0.34 m³/min) with a coarse bubble diffusergenerating bubbles in the range from about 5 mm to 25 mm in nominaldiameter. The air is sufficient no only for the oxidation requirementsof the biomass but also for adequate scrubbing. The fibers have anoutside diameter of 1.7 mm, a wall thickness of about 0.5 mm, and asurface porosity in the range from about 20% to 40% with pores about 0.2μm in diameter, both latter physical properties being determined by amolecular weight cut off at 200,000 Daltons. The skein which has 1440fibers with a surface area of 12 m² is wall-mounted in the tank, thevertical spaced apart distance of the headers being about 1% less thanthe length of a fiber in the skein. The opposed ends of the fibers arepotted in upper and lower headers respectively, each about 41 cm longand 10 cm wide. The fixing material of the headers is an epoxy having ahardness of about 70 Shore D with additional upper and lower laminae ofsofter polyurethane (about 60 shore A and 30 Shore D respectively) aboveand below the epoxy lamina, and the fibers are potted to a depthsufficient to have their open ends protrude from the bottom of theheader. The average transmembrane pressure differential is about 34.5kPA (5 psi). Permeate is withdrawn through lines connected to thecollection pan of each header with a pump generating about 34.5 kPa (5psi) suction. Permeate is withdrawn at a specific flux of about 0.7lr²h/kPa yielding about 4.8 l/min of permeate which has an averageturbidity of <0.8 NTU, which is a turbidity not discernible to the nakedeye.

EXAMPLE 2

[0165] Comparison of Operation of a Vertical Skein (ZW 72) in DifferentOrientations

[0166] In the following comparison, three pairs of identical skeins withequally slack fibers are variously positioned (as designated) aboveaerators in a bioreactor. Each pair is subjected to the same dischargeof air through identical aerators. Rectangular but not square headersare chosen to determine whether there is a difference between each oftwo flat horizontal orientations, which difference would not exist in ahorizontal skein with cylindrical headers. A pair of identicalrectangular skeins, each having headers 41.66 cm (16.4 in) in length(x-axis), 10.16 cm (4 in) in width (y-axis) and 7.62 cm (3 in) in height(z-axis), in Which are potted 1296 Zenon® MF200 microfiltration fiberspresenting a nominal fiber surface area of 625 m², were tested in threedifferent orientations in a bioreactor treating domestic wastewaters.The fibers used are the same as those used in Example 1 above. Thedistance between opposed faces of headers is 90 cm (35.4 in) which isabout 2% less than the length of each fiber potted in those headers.

[0167] In a first test, the two (first and second) skeins were stackedlaterally, each in the same direction along the longitudinal axis, witha 2.5 cm (1 in) thick spacer between the headers, the headers of eachskein being in a horizontal flat orientation (area 41.66 cm×7.62 cm) isspaced apart 7.62 cm (3 in) above the floor on which lies the aeratorsin the form of three side-by-side linear tubes with 3 Mm (0.125″)openings. The first skein which is directly above the aerators istherefore referred to as the “lower skein”.

[0168] In a second test, the same first and second skeins are eachrotated 90° about the longitudinal x-axis and placed contiguously onebeside the other. In this “horizontal 90°” orientation (area defined by10.16 cm×7.62 cm) is spaced apart from the aerators as in the priortest.

[0169] In a third test, the first and second skeins are placedside-by-side in vertical orientations as shown in FIG. 9 except there isno internal aerator.

[0170] Each test provides the fibers in each orientation with theidentical amount of air. Permeate was withdrawn with a pump with a NPSHof 0.3 bar (10″ of Hg). The conditions were held constant until it wasobserved that the flux obtained for each test was substantiallyconstant, this being the equilibrium value. After this occurred, eachskein was back pulsed for 30 sec with permeate every 5 minutes tomaintain the flux at the equilibrium value.

[0171] The test conditions for each of the above three runs were asfollows: TSS in 8 g/L; Temperature of biomass   19° C. bioreactor Flowrate 0.2124 m³/min/ Suction on fibers 25.4 cm of Hg of air skein;

[0172]FIG. 18 is a bar graph which shows the average flux over a 24 hrperiod for each orientation of the skein as follows: Orientation Averageflux L/m2_(/hr over 24 hr) Horizontal flat 21.2 LMH Horizontal 90° 17.8LMH Vertical 27.7 LMH

[0173] This conclusively demonstrates that the vertical orientation ofthe skein fibers produces the highest overall flux.

EXAMPLE 3

[0174] Comparison of Positions of Aerator Inside and Outside the SkeinFibers:

[0175] In this test the difference in flux is measured in a bioreactortreating wastewater contaminated with ethylene glycol, the differencedepending upon how a single cylindrical vertical skein (ZW 172) having anominal surface area of 16 m² is aerated with 3.5 L/min (7.5 scfm). Theskein is formed as shown in FIG. 16 around a central PVC pipe having ano.d. of 7.5 cm, the fibers being disposed in an annular zone around thecentral support, the radial width of the annular zone being about 75 cm,so that the o.d. of the skein is about 11.25 cm.

[0176] In a first test, air is introduced within the skein; in a secondtest, air is introduced around the periphery of the skein. Afterequilibrium is reached, operation is typically continued by back pulsingthe skein with permeate at chosen intervals of time, the intervaldepending upon how quickly the fibers foul sufficiently to decrease theflux substantially.

[0177] The process conditions, which were held constant over the periodof the test, were as follows: TSS 17 g/L; Temperature of biomass 10.5°C. Flow rate 0.2124 m³/min; Suction on fibers 25.4 cm of Hg of air

[0178] For external aeration: A perforated flexible tube with holesabout 3 mm in diameter spaced about 2.5 cm apart was wrapped around thebase of the ZW 72 skein and oriented so that air is discharged in ahorizontal plane, so that bubbles enter laterally into the skein,between fibers. Thereafter the bubbles rise vertically through the skeinfibers. Lateral discharge helps keep the holes from pluggingprematurely.

[0179] For internal aeration: The central tubular support was used asthe central air distribution manifold to duct air into five 4″ lengthsof ¼″ pipe with ⅛″ holes at 1″ intervals, plugged at one end, in openflow communication with the central pipe, forming a spoke-like spargerwithin the skein, at the base. The number of holes is about the same asthe number in the external aerator, and the flow rate of air is thesame. As before the holes discharge the air laterally within the skein,and the air bubbles rise vertically within the skein, and exit the skeinbelow the upper header.

[0180]FIG. 19 is a plot of flux as a function of time, until the fluxreaches an equilibrium value. Thereafter the flux may be maintained byback pulsing at regular intervals. As is evident, the equilibrium fluxwith external aeration is about 2.6 LMH, while the flux with internalaeration is about 9.9 LMH which is nearly a four-fold improvement. Fromthe foregoing it will be evident that, since it is well-known that fluxis a function of the flow rate of air, all other conditions being thesame during normal operation, a higher flux is obtained with internalaeration with the same flow of air.

EXAMPLE 4

[0181] Comparison of Skeins in Which One has Swayable Fibers, the OtherDoes Not:

[0182] The slackness in the fibers is adjusted by decreasing thedistance between headers. Essentially no slack is present (fibers aretaut) when the headers are spaced at a distance which is the same as thelength of a fiber between its opposed potted-ends. A single ZW 72 skeinis used having a nominal surface area of 6.7 m² is used in each test, ina bioreactor to treat wastewater contaminated with ethylene glycol.Aeration is provided as shown in FIG. 9 (no internal aeration) withlateral discharge of air bubbles into the skein fibers through whichbubbles rose to the top.

[0183] In the first test the headers are vertically spaced apart so thatthe fibers are taut and could not sway.

[0184] In the second test, the headers were brought closer by 2 cmcausing a 2.5% slackness in each fiber, permitting the slack fibers tosway.

[0185] As before the process conditions, which were held constant overthe period of the test, were as follows: Suspended 17 g/L Temperature ofbiomass 10.5° C. solids Flow rate 0.2124 m³/min; Suction on fibers 25.4cm of Hg of air

[0186]FIG. 20 is a plot of flux as a function of time, until the fluxreaches an equilibrium value. Thereafter the flux may be maintained byback pulsing at regular intervals as before in example 3. As is evident,the equilibrium flux with no swayability is about 11.5 LMH, while theflux with 2.5% slack is about 15.2 LMH, which is about a 30%improvement.

EXAMPLE 5

[0187] Filtration of Water With a Vertical Cylindrical Skein to ObtainClarity:

[0188] A cylindrical skein is constructed as in FIG. 16 with Zenon®MF200 fibers 180 cm long, which provide a surface area of 25 m² incylindrical headers having a diameter of 28 cm held in end-caps havingan o.d. of 30 cm. Aeration is provided with a spider having perforatedcross-arms with 3 mm (0.125″) dia. openings which discharge about 10liter/min (20 scfm, standard ft³/min) of air. This skein is used in fourtypical applications, the results of which are provided below. In eachcase, permeate is withdrawn with a centrifugal pump having a NPSH ofabout 0.3 bar (10″ Hg), and after equilibrium is reached, the skein isbackflushed for 30 sec with permeate every 30 min.

[0189] A. Filtration of Surface (Pond) Water having 10 mg/L TSS.

[0190] Result—permeate having 0.0 mg/L TSS is withdrawn at a rate of2000 liters/hr (LPH) with a turbidity of 0.1 NTU. A “5 log” reduction(reduction of original concentration by five orders of magnitude) ofbacteria, algae, giardia and cryptosporidium may be obtained, thusproviding potable water.

[0191] B. Filtration of Raw Sewage with 100 mg/L TSS:

[0192] Result—permeate having 0.0 mg/L suspended solids is withdrawn ata rate of 1000 LPH (liters/hr) with a turbidity of 0.2 NTU. Plural suchskeins may be used in a bank in the full scale treatment of industrialwastewater.

[0193] C. Filtration of a mineral suspension containing 1000 mg/L TSS ofiron oxide particles:

[0194] Result—permeate having 0.0 mg/L suspended solids is withdrawn ata rate of 3000 LPH (liters/hr) with a turbidity of 0.1 NTU. High flux ismaintained with industrial wastewater containing mineral particles.

[0195] D. Filtration of fermentation broth with 10,000 mg/L bacterialcells:

[0196] Result—permeate having 0.0 mg/L suspended solids is withdrawn ata rate of 1000 LPH (liters/hr) with a turbidity of 0.1 NTU, The brothwith a high biomass concentration is filtered non-destructively to yieldthe desired permeate, as well as to save living cells for reuse.

EXAMPLE 6

[0197] Special Purpose Mini-Skein:

[0198] The following examples illustrate the use of a mini-skein fortypical specific uses such as filtration of (i) raw sewage to obtainsolids-free water samples for calorimetric analyses, (ii) surface waterfor use in a recreational vehicle (“camper”) or motor home, or (iii)water from a small aquarium for fish or other marine animals.

[0199] A cylindrical mini-skein is constructed as shown in FIG. 16, withcylindrical headers having an o.d. of 5 cm (2″) and a thickness of 2 cm(0.75″) with 30 fibers, each 60 cm long to provide a surface area of 0.1m². The skein is mounted on a base on which is also removably disposed ablower to discharge 15 L/min of air at 12 kPa (3 psig) through a spargerwhich has 1.6 mm (0.0625″) openings, the air flowing through the skeinupwards along the fibers. Also removably mounted on the base is aperistaltic pump which produces a vacuum of 0.3 bar (10″ Hg). In eachapplication, the self-contained skein with integral permeate pump andgas-discharge means, is placed, for operation, in a cylindricalcontainer of the substrate to be filtered.

[0200] The results with each application (A)-(D) are listed below:

[0201] (i) Raw sewage contains 100 mg/L TSS; permeate containing 0.0mg/L TSS having a turbidity of 0.2 NTU, is withdrawn at 0.1 LPH.

[0202] (ii) Aquarium water withdrawn contains 20 mg/L TSS, includingalgae, bacteria, fungus and fecal dendritus; permeate containing 0.0mg/L TSS having a turbidity of 0.2 NTU, is withdrawn at 0.1 LPH.

[0203] (iii) Pond water withdrawn contains 10 mg/L TSS; permeatecontaining 0.0 mg/L TSS having a turbidity of 0.2 NTU, is withdrawn at0.1 LPH.

[0204] It will now be evident that the membrane device and basicseparation process of this invention may be used in the recovery andseparation of a wide variety of commercially significant materials, someof which, illustratively referred to, include the recovery of water fromground water containing micron and submicron particles of siliceousmaterials, preferably “gas scrubbing” with carbon dioxide; or, therecovery of solvent from paint-contaminated solvent. In eachapplication, the choice of membrane will depend upon the physicalcharacteristics of the materials and the separation desired. The choiceof gas will depend on whether oxygen is needed in the substrate.

[0205] In each case, the simple process comprises, disposing a skein ofa multiplicity of hollow fiber membranes, or fibers each having alength >0.5 meter, together having a surface area >1 m², in a body ofsubstrate which is unconfined in a modular shell, so that the fibers areessentially restrictedly swayable in the substrate. The substrate istypically not under pressure greater than atmospheric. The fibers have alow transmembrane pressure differential in the range from about 3.5 kPa(0.5 psi) to about 350 kPa (50 psi), and the headers, the terminalportions of the fibers, and the ends of the fibers are disposed inspaced-apart relationship as described herinabove, so that by applying asuction of the aft face of at least one of the headers, preferably both,permeate is withdrawn through the collection means in which each headeris mounted in fluid-tight communication.

[0206] Having thus provided a general discussion, and specificillustrations of the best mode of constructing and deploying a membranedevice comprising a skein of long fibers in a substrate from which aparticular component is to be produced as permeate, how the device isused in a gas-scrubbed skein, and having provided specific illustrativesystems and processes in which the skein is used, it is to be understoodthat no undue restrictions are to be imposed by reason of the specificembodiments illustrated and discussed, and particularly that theinvention is not restricted to a slavish adherence to the details setfor the herein.

We claim:
 1. An apparatus for withdrawing filtered permeate from asubstrate, including a reservoir containing substrate at ambientpressure; a plurality of membrane assemblies, each assembly having, aplurality of hollow fiber filtering membranes, immersed in thereservoir; at least one permeating header having the membranes sealinglysecured therein; and, a permeate collector to collect the permeate,sealingly connected to the at least one permeating header and in fluidcommunication with lumens of the membranes; one or more sources ofsuction in fluid communication with the lumens of the membranes of eachassembly through the permeate collectors of each assembly and operableto apply sufficient suction to withdraw permeate from the lumens of themembranes; and, an aeration system for discharging bubbles to assist inkeeping the membranes clean, the aeration system including a pipe withholes for discharging bubbles which contact the membranes, the pipeoriented generally vertically above the first header and located withinthe plurality of membranes, wherein the membranes of each assemblyextend generally vertically upwards from a first header duringpermeation.
 2. The apparatus of claim 1 wherein the first header is apermeating header.
 3. The apparatus of claim 1 wherein the pipe isembedded in the first permeating header and the membranes are secured inthe first permeating header around the pipe.
 4. The apparatus of claim 1wherein the holes in the pipe include holes spaced around thecircumference of the pipe and holes spaced along the length of the pipe.5. The apparatus of claim 1 wherein the first header is circular in planview and the pipe is located on a vertical line through the center ofthe first header.
 6. An apparatus for withdrawing filtered permeate froma substrate, including a reservoir containing substrate at ambientpressure; a plurality of membrane assemblies, each assembly having, aplurality of hollow fiber filtering membranes immersed in the reservoir;at least one permeating header having the membranes sealingly securedtherein; and, a permeate collector to collect the permeate, sealinglyconnected to the at least one permeating header and in fluidcommunication with lumens of the membranes; one or more sources ofsuction in fluid communication with the lumens of the membranes of eachassembly through the permeate collectors of each assembly and operableto apply sufficient suction to withdraw permeate from the lumens of themembranes; and, an aeration system having openings of air passages atfirst ends of through-passages, the openings located to dischargebubbles which contact the membranes, the through-passages passingthrough the first header, wherein the membranes of each assembly extendgenerally vertically upwards from a first header during permeation. 7.The apparatus of claim 6 wherein second ends of the through-passages arein communication with a source of pressurized air.
 8. The apparatus ofclaim 6 wherein the openings are located to discharge bubbles directlyinto the substrate between the membranes.
 9. The apparatus of claim 6wherein the first header is a permeating header.
 10. An apparatus forwithdrawing filtered permeate from a substrate, including a reservoircontaining substrate at ambient pressure; a plurality of membraneassemblies, each assembly having, a plurality of hollow fiber filteringmembranes, immersed in the reservoir; at least one permeating headerhaving the membranes sealingly secured therein; and, a permeatecollector to collect the permeate, sealingly connected to the at leastone permeating header and in fluid communication with lumens of themembranes; one or more sources of suction in fluid communication withthe lumens of the membranes of each assembly through the permeatecollectors of each assembly and operable to apply sufficient suction towithdraw permeate from the lumens of the membranes; and, an aerationsystem for discharging lines of bubbles from openings near the firstheaders, the lines of bubbles separated by 30 fibres or less, whereineach assembly has at least a first header and the membranes extendgenerally vertically upwards from the first header during permeation.11. An apparatus for withdrawing filtered permeate from a substrate,including a reservoir containing substrate at ambient pressure; aplurality of membrane assemblies, each assembly having, a plurality ofhollow fiber filtering membranes, immersed in the reservoir; at leastone permeating header having the membranes sealingly secured therein;and, a permeate collector to collect the permeate, sealingly connectedto the at least one permeating header and in fluid communication withlumens of the membranes; and, one or more sources of suction in fluidcommunication with the lumens of the membranes of each assembly throughthe permeate collectors of each assembly and operable to applysufficient suction to withdraw permeate from the lumens of themembranes, each assembly having at least a first header which iselongated in plan view, the membranes extending generally verticallyupwards from the first header during permeation, the first headers ofthe assemblies being located in a spaced side by side relationship; and,an aeration system having through-passages for discharging lines ofbubbles between and at the sides of the elements from openings locatednear the first headers and, there being n+1 lines of bubbles for nelements.
 12. The apparatus of claim 11 wherein, the lines of bubblesare separated by 30 fibres or less.
 13. An apparatus for withdrawingfiltered permeate from a substrate, comprising: a header having a firstface and a second face; a receptacle for collecting permeate, thereceptacle being in fluid communication with the second face of theheader and having a permeate outlet; a plurality of hollow fibremembranes, the hollow fibre membranes sealingly secured in the headerand protruding from the first face of the header, and having ends opento the receptacle for collecting permeate such that a portion of thesubstrate drawn into the lumens of the fibres as permeate may flow intothe receptacle; and, a pipe with holes for discharging bubbles, theholes located near the first face of the header, the pipe protrudingfrom the first face of the header from a location within or adjacent tothe plurality of membranes.
 14. The apparatus of claim 13 wherein thepipe protrudes from the first face of the header from a location withinthe plurality of membranes.
 15. The apparatus of claim 13 wherein thepipe is located axially within a plurality of membranes that surroundthe pipe.
 16. The apparatus of claim 13 wherein the membranes aregenerally parallel to each other and the pipe is generally parallel tothe membranes.
 17. The apparatus of claim 13 wherein the holes arelocated at various locations around the circumference of the pipe. 18.The apparatus of claim 13 wherein the pipe has holes for dischargingbubbles at a plurality of locations along the length of the pipe. 19.The apparatus of claim 13 wherein the holes are located in armsextending outwards from the pipe.
 20. The apparatus of claim 16 whereinthe first face of the header is circular and the pipe protrudes from thecenter of the first face of the header.
 21. The apparatus of claim 20wherein the pipe is secured in and extends through the header.
 22. Theapparatus of claim 16 wherein the membranes and pipe protrudeperpendicular to the first face.
 23. A system for withdrawing filteredpermeate from a substrate, comprising: a reservoir containing asubstrate at ambient pressure; an apparatus of claim 13 immersed in thesubstrate such that the first face of the header is generally horizontaland the membranes extend generally vertically upwards from the firstface of the header; a source of suction in fluid communication with thepermeate outlet and operable to apply sufficient suction to withdrawpermeate from the lumens of the membranes; and, an aeration system forsupplying a pressurized gas to the pipe.
 24. The apparatus of claim 13immersed in a substrate at ambient pressure.
 25. The apparatus of claim24 wherein the membranes extend along their length generally verticallywhen in use and bubbles discharged from the holes rise to contact themembranes after being discharged into the substrate.